Twist rate, heat treating & powder selection

[offshore44]

I have a Yugo M24/47 that shoots great with a 190 grn Saeco cast from Lyman #2. The mold is a single cavity. I've been messing about with loads and powders a bit. 32 grns of H4895 with no filler is my current "go-to" load. Very consistent and surprisingly accurate for a beater mil-surp. The twist rate is 1:9.4.

I have been experimenting with it some to see just what it is capable of. Beagled the mold out a hair to increase the driving band size to fit the throat, made 7.92x57 cases out of 30-06 cases to get a little thicker necks, that sort of thing.

Now I want to see just how fast I can drive these things and still maintain accuracy. Don't need to, but it seems like the thing to do at the moment.

I heat treated some 94-3-3 alloy up to about 29 bhn. Anyone have any ideas how fast these can be pushed before things go south? The 32 grn load of H4895 is already pushing these over the"RPM Threshold" by a bit with good success, so a lot of things are working well already. Any ideas how far I can push my luck with heat treating?

As an aside, changing the oven temp allowed me to get 17 bhn, 22 bhn and 29 bhn out of the same alloy. Consistently. Water dropping gave me inconsistent hardness's (strengths). They were pretty much all over the map from about 18 to 28 bhn. Lots of variables there to control.

Thanks for the input!

[popper]

Drop most of the tin and add 1% copper. 2400+ fps (~40 H4895) in a 1:10 308 is way over the RPM factor, still MOA when I do my part. H.T is really neat.

[45 2.1]

Anyone have any ideas how fast these can be pushed before things go south? The 32 grn load of H4895 is already pushing these over the"RPM Threshold" by a bit with good success, so a lot of things are working well already.

Full jacketed velocity for that boolit weight. There is no "threshold" when you load it correctly. I would dump that powder and go to a lot slower powder myself.

[runfiverun]

ditto the slower powder.
2 steps slower than normal jaxketed powder is a good direction to take.
maintain the case anneal [not dead soft] and find the center line of the barrel with your boolits centerline.
another thing to look at is your neck tension [part of the anneal] but .0015 is a pretty good place to start.

[Larry Gibson]

As we continually see there is still a lot of misconception about the RPM Threshold. The RPM threshold aplies to generic naked GC'ed cast bullets of AC'd ternary alloy (Pb, Sn, Sb). The threshold is most often found in the 120,000 - 140,000 RPM range. The RPM threshold is simply that point where accuracy begins to deteriorate due to the centrifugal force overtly acting on the imbalances in the bullet. The loss of accuracy is most often charactorised by flyers initially and by non linear group disersion as the range increases. The RPM threshold has nothing to do with the bullets stability in flight or with surface tension, aka "blow up".

The above RPM range is also Not a limit. The RPM attained before accuracy declines can be raised by what the OP is doing (loading technique, hardening the bullets, slower burning powders, etc.) or it can also be lowered. The hardened bullets should do well as should the adding of copper. I also suggest slower burning powders such as RL19, AA4350, H4831SC and RL22 be tried.
I do have to question; "The 32 grn load of H4895 is already pushing these over the"RPM Threshold" by a bit ". I shoot quite a few 190 gr cast in my own M24/47 and M48. I shoot a lot of H4895 powder under that bullet. I think you will find, if you chronograph your load that it is running perhaps 1600-1700 fps, hardley "over the RPM Threshold". If guestimating the velocity you might check Lyman's #4 CBH where you'll find 35.5 gr H4895 is only pushing that Saeco 190 gr bullet 1700 fps.

Larry Gibson

[offshore44]

Yup, Lyman#4, page 176, 35.5 grn of H-4895 1700 fps. Thanks for catching that Mr. Gibson. I stand corrected. The Chronograph absorbed a 230 grn 45 ACP round a few months back, so....

I think I am starting to understand the "RPM Threshold" theory. I understand that it is not like a speed limit or some-such. It is the broad range where you need to start paying close attention to the details and uniformity for good results. Am I starting to see the light there?

The suggestions are so far:

Reduce the tin content and add copper to the alloy...

Boolit / bore alignment, proper anneal on case necks, and 0.0015" neck tension on the cases with properly fitted boolits.

Two steps slower powder than recommended jacketed powder. Also RL19, AA4350, H4831SC and RL22 powders.

I can work on Boolit / bore alignment, proper anneal on case necks, and 0.0015" neck tension on the cases with properly fitted boolits while re-reading the copper alloy threads and searching for a pound of slower powder. I think that the slowest powder that I currently have on hand is what is left of a pound of IMR4350. I'll give that a try whilst scrounging up other powders.

Heat treating is pretty cool. This is the first time that I've tried it more than casually with water dropping.

Thanks folks!

Edited to add: Any recommendations on a starting load for IMR4350? Not seeing a lot of load data out there for this combination of cartridge, bullet weight and powder. Ahhh...Found it! Sierra load data. IMR4350 and the 200 grn Matchking. Start load of 45.4 grns and max load of 48.7 grns. Velocity from 2000 to 2300 fps for the Matchkings.

[runfiverun]

now you are getting it.
a buffer might help things along too, it helps protect the base of the boolit.
I'd start with what you have listed first just to get a handle on things, then slowly work forward.

[geargnasher]

ditto the slower powder.
2 steps slower than normal jaxketed powder is a good direction to take.
maintain the case anneal [not dead soft] and find the center line of the barrel with your boolits centerline.
another thing to look at is your neck tension [part of the anneal] but .0015 is a pretty good place to start.

You know, it took me nine full essays to just scratch the surface of the things you just captured in three sentences. Technical writing does tend to get a bit long on the specifics, but I've always stru
ggled to make concise, academic introductory paragraphs.

Anyway, for those who may have missed it, it will probably take you about six months to a year of regular shooting to fully understand and accomplish what Run mentioned above, so study hard. Do those things correctly, plus a couple others, and you won't have to worry much about any rpm issues. This ain't in the Lyman Cast Bullet manual.

One more thing, the copper alloy may not really be neede until 22-2300 fps.

Gear