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.

Wrong.

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.

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:

N=RPM

A=MOMENT OF INERTIA IN SPIN

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:

S=A^2*N^2/(B*U)

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.

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.

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:

Results:

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.