Monday, November 17, 2014

Cadence and the Crack Shot

A few matches ago, I noticed a phenomenon in my shooting.  Every so often, I would notice my shots starting to move.  I’d make a small sight change and immediately see a large motion in the hit location.  I always passed it off as a possible issue with my sight.  Then I was reading a book on high power shooting by G David Tubb.  He talked about maintaining a steady cadence and the idea that heating of the cartridge in the chamber can change the muzzle velocity.  My thought was that if I chamber a cartridge before fiddling with the sights, then that one shot would heat soak in the chamber for longer than all the other shots.  This would throw the shot high, or at least wild.
So, off for a test.  I made up 20 test rounds.  I set up 2 targets side by side.  After shooting a number of rounds to warm the barrel, I then shot at the 2 targets alternating right and left.  Every shot to the left I shot at normal cadence.  Shots to the right I sat on for a measured 30 seconds, then shot at my regular cadence.  My expectations were a similar group size for each style with the slow cadence side grouping higher than the fast cadence.


The variation in elevation of the 2 groups was .06 inches at 200 yards.  Essentially nothing.  However, group sizes were very different.  Fast cadence had a standard deviation of 4.27  while the slow cadence had a standard deviation of 2.99.  This is a big chunk.  Both of these might be a bit higher than normal because I went about 10 shots longer before cleaning than normal.

The next question was what the shortest reasonable wait time would be.  I wired up a cartridge with a thermocouple run in through the primer hole.  I built it up with powder and a bullet, just like the real thing.  Then, after each string at a match, I’d pop the wired shell in and record the temperature rise over time.  

The shell is a bit dirty, but I've been pushing it into dirty chambers all day.

The chart below shows the various data strings.  The temperature was rising throughout the day, so the starting point is a bit off for each.  I also was a little slapdash on getting started promptly when the shell went in.  Overall, it was a bitterly cold day.
 Temperatures are degrees F and time in seconds.

Clearly the heating steadies out after about 30 seconds.  My normal cadence would have a cartridge in place for 10-15 seconds, so there was probably a lot of variation at that pace.

I'll probably do a similar test on a hot day, but for now, I'll just be patient when I need to take that Crack Shot.

Saturday, November 15, 2014

End-Over-End me through trouble and strife.

I’ve recently added to my monthly 200 yard silhouette match.  I’ve gone out to a mid and long range match starting at 300 and going out to 1000 yards.  Now I’ll admit, I don’t use the 38-55 Trapdoor beyond 600 yards, but it does get to reach out a bit.  Besides the eye opening challenge of shooting long range in a 25MPH side wind, I discovered an additional issue.  My .38 bullets seem to be going unstable at the longer range.  I knew they were OK at 300 yards because I can do a lot of testing at that range locally, but 600 yards is harder for me to get time at.
A book was suggested to me.  “Understanding Firearm Ballistics” by Robert Rinker.  It was pretty straight forward and came with one formula on basic stability that would help me compare my current “OK” bullet with any new bullet I want to try.  The formula basically compares the moment of inertia of the bullet in the spin direction with the moment of inertia in the tumble direction.  It also has a term for the RPM of the bullet spin, which turns out to be a fairly heavy player.
So labeling:
B=MOMENT OF INERTIA IN TUMBLE (OR YAW-it should be the same either way).
S=”STABILITY”.  I will be treating it as a dimensionless number as long as all my input numbers use the same units from one bullet to the next.
U= A shape factor.  I have no way of calculating this, so I choose to ignore it.  Easy, eh?

The basic formula is:

I apologize for my poor math text layout. 
The important thing to notice is that stability goes up as the square of the moment of inertia in spin and the square of velocity.  So a short squat fast bullet should be highly stable.

The next step is to compare my current bullet with my new bullet.  A little time with a cad program and some calipers and I had my numbers.  The pictures below show my two bullets.

My current bullet is a Postell style that weighs 336 grains. It is a Lyman 378674DV
The Round Nose Flat Point weighs 305 grains.  It is a Redding Saeco 65571  #571.
The Postell has a CG that is 6.3% of the length back of the center.  The RNFP CG is 4.1% back of the center.  This in itself will add some stability as the tendency of the nose to lift is reduced.
The Postell “A” value is .0007675 LB*in*in compared to .0007279 for the RNFP. 
The Postell “B” value is .005918 as compared to .004374 for the RNFP.

So, assuming the muzzle velocity is unchanged (which, sadly, seems to be the truth despite the lower weight) I get an increase of stability of right at 20%.  Which is sizeable, but is it enough?
Well, off to the range.  At 300 yards, the new, shorter, bullet throws a 10 shot group with a standard deviation of 1.56.  The older, postell, bullet throws a standard deviation of 3.76.  This is a big chunk and the smaller bullet has become my baseline.  However, at 300 yards, it doesn’t really answer my question.  Is it more stable at 600 yards?  Unfortunately, the data acquisition at 600 is a little tougher.  In matches, each shot is marked by a target puller and can be viewed from a scope, but that’s a lot less accurate than measuring paper.  So my answer right now is “I don’t know.”  Hopefully, sometime in the spring, I can get to a test range during low wind and shoot some test targets.

Thursday, January 23, 2014

Trapdoor Springfields can't Twerk

Some of you may not know this, but I get a bit of data about what you're searching for before you click on this blog.  Some people have quite reasonable searches like "broken trapdoor firing pin"  or "trapdoor springfield accuracy".  Sometimes, I can tell the search engine has led them astray.  My blog about using an old microscope box as an ammo carrier is immensely popular, and yet, I feel people were actually looking for microscope cases and wound up here by accident.

I recently saw a search that set me back a bit.  It was "Trapdoor Springfield Sucks".

I am well aware of the trend to add "sucks" to any subject to try to find a site dedicated to a subject's belittlement.  I'm sure "Justin Bieber Sucks" or "Miley Cyrus Sucks" will bring up plenty of fodder about poor singing, poor life choices, and mediocre twerking.  But "Trapdoor Springfield sucks?"  Surely there isn't a trapdoor hate site?  Well, where there is a need, there is a supplier.  What follows is a list of the trapdoor's failings, in my experience.

1)  It was built to a price point.  There is no doubt, that when the government started with the Allin Conversion, the main goal was to use up all of the muzzle loaders they had sitting in the armory.  The ability of the trapdoor concept to use up these free parts was surely a major selling point.  After that, even when making rifles from scratch, the trapdoor is a simple piece to make.  Remember that the trapdoor was never the active arm during any major conflict.  It is hard to spend money on rifles during peacetime.
2)  The rifling is off.  I have to agree on this one.  Current BPCR guns use 18:1 twist for a reason.  22:1 is surely better than the 4 foot twist of some muzzle loaders, but still not quite there.  I can't think of a good reason why 3 lands is worse than any other number, but people have surely landed on 6-8, so there may be something to it.
3)  The trapdoor barrel is too whippy.  Sure enough, a rifle built to be carried across America on foot or horse is not going to have the heavy bull barrel of a target rifle.  Plus, steel costs money, see #1.
4)  It is unreliable.  I call this one false.  The troubles of Custer are well documented and certainly stem from the balloon cases of the time.  In my 1000s of rounds I have had zero fail to fires and one case failure that didn't stop it from functioning.  Even my broken firing pin was still functioning when I changed it.
5)  The trapdoor is weak.  I give this a 50%.  the trapdoor is fully capable of supporting the round it was designed for.  The fact that technology marched on and created incredible pressures in the 45/70 doesn't change the rifle.  The same can be said of all the straight walled black powder cases.  This is why the revolvers tended toward a longer, high pressure version.  .38 to .357 mag.  45 to 454 Casull. 44 to 44 mag, etc. 

So there you have it.  The truth is out.  The Trapdoor Springfield sucks and there's no way to deny it.

It also can't twerk worth a damn.

Sunday, January 19, 2014

Powder Testing. A Pain in the Brass.

After my recent experience where FFF powder worked so poorly, I figured the old girl was sensitive to powder type, so I set about testing.  I ordered up a variety pack of different brands and granulations.  So far, I've tested about 6 versions other than my standard FF KIK.  I did all the tests with the same 40 grains and the same overall length.  This means some powders had more compression than others.  Obviously, there is a lot more testing to do. 

There are 3 competing requirements in the load.  One is that the initial pressure needs to be strong enough to upset the bullet into the rifling, without upsetting it too much and deforming it out of round or leading the barrel.  Two is that the combustion needs to be regular, steady and clean.  Lastly, the final muzzle velocity needs to be in a range where the bullet is happy aerodynamically. 
There are lots of ways to play around with this.  More powder raises muzzle velocity and the initial upset pressure.  It can make a burn more regular, or more erratic, it just needs testing. 
Coarser powder decreases the initial pressure, and might decrease muzzle velocity, but with my long barrel it may not affect velocity much. 
Balancing off a powder change with an amount change could give me exactly what I need, or just the opposite.

All of these options can have base wad thickness modified to affect compression.  There are at least 100s of possibilities.  On top of all that, a slight change in bullet hardness might alter the results concerning the initial pressure.

So, a pretty picture:


All tests were at 200 yards, bench rest, 320 grain 30:1 bullet, .060 wad, 40 grains measured, very light compression that varied by load to meet the same OAL.
For the data below I'll give maximum group size and then "9 shot" group size.  A big difference between the 2 indicates a "flier".  I'm not much into calling fliers, as they are still a miss, but the existence or lack of them can point at combustion problems, or bullet quality control issues.
In none of these did I run a chronograph, so we'll need to hold that for a later day.

Powder                         10 shot group    9 shot group.
KIK FF                         9.3                     6.6
KIK F                           8.5                     6.9
Swiss  1.5F                   8.8                     6.8
Olde Eynsford 1.5F      7.2                     6.9
Olde Eynsford  FF        5.2                     4.8
Goex F                          10.1                   8.9
Goex FF                        6.6                     5.6

I should comment that the first load, the KIK FF is my standard go-to load (or it was until I saw the Olde E result), and usually shoots about 7.5 inch groups.  No excuses, a test is a test, but it shows how variable a single round of testing can be.

That Olde Eynsford FF is looking pretty good right now...

Calipers are set at 5 inches.

In the match today it was rather foggy, so my cross stick shooting was not up to my bench rest results in the sunshine.  I did, however, have a bit of excitement.  Near the end of practice, this ejected:

The shell on the left is for comparison.

The trapdoor is a very difficult gun to inspect.  After trying to push the chunk out with wads, we ended up driving a bullet down from the muzzle.  The spooky thing, nothing came out but the bullet.  It seems the ring of brass formed itself into a crude gas check and went out with the original shot (high and right, by the way).  I've only seen one other case like this in a BPCR, and it left the ring in the chamber.  There were no unusual effects, and the reline job didn't give way, so I'm still happy, if concerned.   A thorough brass inspection is in the works.  Cracked brass can be an indication of headspace issues, which I'm not expecting, but I'll check.  Usually a headspace crack is closer to the base.
Have fun.  Be safe.

Sunday, November 17, 2013

5 Will Get You 10

It has been a while!  There are a few reasons for this.  I didn’t want to report back on the 38-55 Trapdoor project until I had some firm results.  I also just plain got busy.
For a long while I couldn’t buy any of the correct brass for this relined barrel.  The 38-55 comes in 2 lengths.  The longer 2.125” is the original standard.  A slightly shorter version came about several decades ago because nobody was making brass and a short brass could be made out of another, similar case.   Anyway, in the great “Obama’s going to steal my brass” hording scare of 2013, I found I was out of luck.  I did manage to find a work around for testing.  It was still possible to get “38 Basic” brass, which is way too long and ungodly expensive.  I picked up 20 cases and cut them down for testing.  Finally, about a month ago, Starline came through with standard cases.
Load testing was interesting.  I still have a long ways to go, but I at least know what not to do.  Like the 45-70, it doesn’t like light bullets.   I’m currently at 320 grains and looking to try bigger.  The higher loads are bad as well.  I’m currently at 44 grains and will be trying less.  I tried 1 batch of FFFG.  Typically, I don’t experience much difference with grain size changes.  On this rifle it made the pattern triple in size.
So today, now that I had some brass and a load, it was time to take the old soldier to a match.  It was a bit breezy, but I can’t really blame that much.  I’m incredibly pleased with how light the recoil is.  As it stands, the gun is shooting as well as my rolling block.  I’m convinced it has a ways to go.
You’re probably wondering by now “Why 5 Will Get You 10?”  Was there some Double-or-Nothing bet we don’t know about?  Take a look at this target.

I shot this today in the match.  5 in the black, and the other 5 floating about 1/4 inch away.  A 5% reduction in group size should do the trick.  Of course, if I'd just adjusted my sights up 1", that would have worked too.  Hopefully, I'll be updating this blog a bit more frequently now that I'm shooting the old girl again.

Sunday, March 31, 2013

38-55 Trapdoor

The main reason I abandoned the trapdoor and got the Pedersoli rolling block was the barrel.   I’m not convinced that the rolling block action is any better than the trapdoor, and in some ways it is worse, but there was no getting around the worn and slightly pitted barrel.  The long term solution was a new barrel.

I pondered four options.  The first was to get an original barrel with crisp rifling.  I found that hard to find, and believed it to be too expensive, though it wasn’t bad compared to my final solution.  The second was to get a new barrel made to the original external shape.  This would allow me to move away from the original three groove, twenty two inch twist if I wanted.  This option is easier to find, but again money held me back.  The third option was to reline the barrel to 45-70.  This could either be the original three groove configuration or something different.  The last option, and the one I chose, was to reline to a different caliber.

The reasoning for the change in caliber was twofold.  The first was strength.  The relining process bores a hole all the way down the barrel and fills it with a thin barrel.  How strong this system is depends greatly on how well the inner liner transmits load to the outer remains of the original barrel.  When I looked at the wall thickness of the liner for the 45-70, I decided I just liked the 38-55 wall thickness better.  Both liners had the same outside diameter.  The second reason was accuracy.  The 38-55 has a fine reputation at medium ranges, and since I normally shoot at 200 yards, it seemed to be the cat’s meow.  A third, more subtle, benefit is reduced recoil.  I’m not much affected by recoil, but it could be a driving factor for some lighter framed shooters.

The  Process:

I’ve never done a reline of a barrel before, but I’ve spun a lathe a bit.  I knew that the #1 problem in most repair jobs is how to accurately and rigidly hold the work piece.  With the outside of the barrel tapered along the full length I knew that no chuck or collet I owned would do the trick.  I opted to pick up 2 “soft” collets.  These are steel, though they also can come in brass or plastic, that is machinable.  I set my lathe to cut the same taper as the barrel in the bore of the collets.   One grips the barrel a few inches from the breech, with the breech sticking out.  This is where 90% of the work is done.  The other grips a few inches from the muzzle for cleaning up the crown.  I will issue a warning that the collet near the breech left a few light scratches that seemed to have been put in just as I was seating the collet.  In my case I was unconcerned because I had bought a crusty rust pile barrel to do this to.  It was going to get a full refinish.  Also, the front collet struggles to get over the front sight.  The removable blade sight needs to be pulled, and a notch cut in the collet.

Before machining can begin, a bit of tool fabrication is required.  The main task is to make a long drill bit.  I purchased a bit with a bore riding pilot from Pacific Tool and Gauge.  It is a well made unit ground to the OD of the liner.  The trouble is that the shank is short.  I took a length of precision ground rod and bored a hole in the end that is a slight press fit for the shank, then pushed it in with Loctite.  Some people claim to weld the extensions on, but I can’t see how there isn’t heat warping involved.  The drill loads seemed very light anyway.
The shot above is the drill being held with the pilot in the chuck for alignment, being pushed onto the end of the steel rod that has been bored in the lathe collet.  The rest of the rod is about 3 feet long and rests inside the lathe spindle.

There are a few accuracy concerns in this job.  The first is that the drilled out bore needs to align with the original barrel for a neat, concentric crown.  The piloted drill pretty much assures this.  The second is that the chamber need to be concentric to the new bore, aligned to the new bore, and of the right dimensions.  Some of this is assured by using a piloted reamer.  The front of the reamer is thus forced to be true to the bore.  The rear of the reamer remains true to the bore solely by dint of all the cutting steps being taken in the same setup.  No removal and realigning of the barrel is required.  It is also a good idea to check alignment of the lathe tailstock in preparation for this job.  If the tailstock is offset, the reamer will be pushed to the side and will cause an excess taper with too large an ID at the breech.  You may also see an uneven amount of cutting on each of the reamer flutes in this situation. 
This shows a tap handle driving the reamer and a dial indicator to show the depth of cut, for when the final headspace is being cut.  Some companies make a floating reamer driver that can be aligned perfectly to the bore.  This is probably a good idea if you were super serious about accuracy.
The last accuracy concern is headspace, the control of the depth of cut.  In a rimmed cartridge, this becomes simply controlling the seating face for the cartridge rim.  You can choose to set per SAMMI specs, which will allow all cartridges to load.  Most target shooters set for minimum clearance on whatever brass they choose to shoot.   I used Starline and found they tended to have thin rims.  This allows me someday to do a cleanup cut on the chamber and use brass with a thicker rim.  It also means some other brass might not allow the action to close right now.
If the old action had good headspace, a simple depth measurement from the end of a seated cartridge to a fixed feature like the thread shoulder is all you need.  If you want a different headspace, some corrections can be made from there. 

The drilling went easily, with the trapdoor barrel being fairly soft and making “crumbly” chips.  The liner was glued in with green “shaft lock” type Loctite.  A little facing was done on the breech, then the chamber was cut.  This was done unpowered using the lathe for alignment.  The final cuts took a few iterations of fitting the breech block to check headspace.  When the barrel is removed and flipped the crown could be cleaned up.  I’m a little displeased with the crown.  When the cutter skims over the lands and grooves, it leaves little uneven burrs that I couldn’t quite control with any of the normal cutter tricks.  I’m also unable to prove the crown is truly concentric with the bore, since the setup had to be dismantled and flipped.  People seem to rave about an 11 degree crown.  It seems to me a true zero degree crown could be machined more reliably, and that might overcome a poorly made 11 degree crown.  Either way, I have ordered an 11 degree piloted hand crowning tool and will give that a try.

One final challenge.  The original extractor is too thin to reach the smaller cartridge rim.  I fabricated a new one and cut the notch for it in the relined breech.
This standard T-slot cutter fit the bill nicely.  I have never had nice "clinky" ejection, even with a 45-70.  I have seen it done and it is pure magic when the indians are raiding.  For target work, I am fine with the more restrained control of spent brass that my trapdoor ejector gives me.
Here's a shot of the 38-55 nestled in the big-bore-breech.  Cute!

Cost totals  (Some are guesses):

Old barrel, receiver, door. $75-100
Liner  $5.39/inch   Track of the
Drill  $75
Shank  $20
Reamer $95
T slot Cutter  $15
Crown tool  $85
Brass  $62
Dies  $50
Headspace gauges - cheap, but a total waste.  In bottleneck cases they measure something difficult to measure and important for safety.  In rimmed cartridges, they are glorified washers, and you should learn basic measurement techniques instead.

How does it shoot?  Well, I can’t let all my secrets out in one blog.   Stay tuned.

Thursday, December 20, 2012

Trapdoor Trigger Pull.

When I got my Pedersoli rolling block, I was astounded at how firm the trigger was.  Taking it apart, the action proved to be very simple, not allowing for a lot of trickery.  Other than bucking up for a complete new double-set trigger setup, there are only 3 things you can do to improve trigger pull.  First, you can thin the mainspring.  This works, but it increases lock time and could lead to intermittent firing of the primer, so I left it alone.  Second, you can change the sear angle.  This also works, but there is a risk of making the sear unsafe if the nock tapers away from the sear.  The gun can just go off by itself because only friction is holding it cocked.  I left this alone as well, except that I do a little polishing to slicken up the feel.  Lastly, you can lighten the trigger return spring.  This spring pushes the sear up into the notch and the worst drawback if you go too far is that the hammer might not "lock back" and you'll set the hammer back slowly on the firing pin.  Once the hammer latches, the trigger return spring is no longer what actively secures the sear, the nock angle is.  So,  I made a new spring out of "safety wire."  This is .032" stainless wire and is significantly softer than the stock spring.

Here you can see the wire wrapped around the mounting screw and resting on the sear.  The original spring was a flat strip that filled the whole width of the trough.

The result was a trigger pull of 36 ounces.  This sounds light, but in the offhand position it feels terrible.

But this is a blog about Trapdoors, right?  Here's a shot of the same concept applied to the Trapdoor lock:

The wire loops around the mounting post and an extra loop slides into the slot that holds the stock spring.

Here you can see that the spring as it pushes on the sear.  It may look like the wire would easily fall out, but the wrap around the post holds it well and when installed, the wood of the stock traps the spring.

How does it work?  Trigger pull is down to 21 ounces and feels very nice.  This is the same setup I used to win the offhand competition a few months ago.  By comparison, my Izhmash Ural 5-1 target .22 is set at a pull of 6 ounces and feels like it isn't there at all.