twist rate, rpm's, and short barrels

Boolit Man Join DateAug 2007LocationMilwaukeePosts140

What would the ideal twist rate be for a 30 caliber barrel in a 10" length? The cartridge would be a 30 herrett and the preferred bullet weight would be 150 grains. The aimed for velocity with accuracy is 1800 - 1900 fps given all the other variables are taken into account.

This is about the max velocity with accuracy for a 1 in 10" in a rifle, at least for most and with relative ease. I'm wondering how this changes in a short barrel because the powder to achieve the same velocity will be much faster than in a rifle as will be the acceleration. Do we need a slower twist just to make the 1800-1900 goal in a 10" barrel?

Don't think it is going to happen in a 10" barrel. Not with cast anyway.

i don't see any reason to use a different twist then you would in an 18 inch barrel. just change up the powder for better performance.

what I hand-load; 45 acp; 223 rem(5.56mm); 22-250 rem; 243 win; 30-30 win; 308 win; 45-70 gov.
on the list to start Loading; 9 mm; 270 win; 30-06 springfield; 222 rem

"You might be a gun nut if you load 45-70 on a progressive press" -HICKOK45<- was he talking about me!?!

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Si vis pacem, para bellum

There are twist rate calculators out there on the web. A shorter barrel may need a faster twist for that velocity for no other reason than the muzzle blast. It's the muzzle blast that destabilized a boolit but how much difference that would make is a guess. The stability twist to velocity is not linear. It becomes iffy at transonic and requires more twist below transonic than above and more twist as velocity approaches 3000fps.

Regards
303Guy

?It is hard enough to remember my opinions, without also remembering my reasons for them!? Nietzsche

The barrel twist is not going to effect the goal of "1800 - 1900" fps much. The standard twist for the 30 Herrett was/is 14" and it is a good one for what you want. However, you can sneak up into the 1700s with a 150 cast with a 30 Herret but you need a longer barrel or the case capacity of the parent 30-30 cartridge to safely get into the 1800 - 1900 fps range.

The older Lyman reloading manuals with the "Contender" data section has loads.

Larry Gibson

The Herrett, formed from 30-30 brass is 1.605" long. Rifling for a 10" barrel is 1 in 14" which works at around 1593 fps with a 150 gr bullet, about the same with a 165 gr bullet.
WW748 is best, and faster like 4227, 4198 also failed for accuracy.
You CAN get to 1850 fps with the 150 jacketed, I have nothing for cast. But accuracy is sad.
A good load is 25.5 gr of 748, 165 gr SPBT at 1525 fps. 27 gr with the 150 gr bullet for 1593 fps.
A faster twist will need LESS velocity, a slower twist needs more but you have pressure limits and burn rates from the short barrel.
I would play with SR4759 for accuracy and then a powder not even thought of, VARGET! You will be on your own with both, not much for loads with either powder. Not many loads for cast either.
Hodgdon will tell you Varget is too slow but I use it in the 7BR and 7R with good velocity and supreme accuracy.
I think your wish for high velocity and accuracy is a step too far, not needed, just accuracy.
I used 4759 in my 30-30 tender but I no longer have the loads, gave them when I sold the gun. I used the RCBS 308-165-sil boolit and could do this at 100 yards from the bench, scoped.

You might also want to check out your chamber's throat or freebore in this case. T/C liked to be very generous in freebore on their pet rounds. I wouldn't be surprised if it was 0.250 to 0.375 off of the case mouth. A friend's 357 Herrett is over 0.250. The long run in makes cast a challange.

You might also want to check out your chamber's throat or freebore in this case. T/C liked to be very generous in freebore on their pet rounds. I wouldn't be surprised if it was 0.250 to 0.375 off of the case mouth. A friend's 357 Herrett is over 0.250. The long run in makes cast a challange.True, they were all over the place and it changed from gun to gun. Rough bores were common too.

A few golden booliters have chimed in on your post. I will not try to compete with their knowledge in this area. It is a interesting concept though. Your OP suggest a custom barrel and experimentation. With the 1 in 10" twist and a WD or heat treated BruceB boolit the pressure should increase along with speed with a good chambering and boolit fit. As for accuracy are you talking scoring target or definitely behind the front shoulder at longer ranges. 44man made a good comment of "but you have pressure limits and burn rates from the short barrel."

When I played with my T/C 10" Herrett, I wound up with around 25gr of AA1680 powder (H4227 gave the highest velocities, but was NOT consistent or accurate), 6.8SPC brass for parent cases (change extractor to one from a 6.8SPC barrel), and managed 2150fps and 1.25" 5 shot groups @ 100 with 125gr Ballistic Tips. I only shot very little cast in it for fire forming, not for accuracy.

The throat in mine was super long. I found case neck tension (having plenty of bullet shank in the case) was more important that trying to approach the rifling, since I could drop a 180gr round nose in and put the case in behind it and it never touched.

In the end I sold it and figured I'd be better off with a .300 whisper or blackout.

I think 1:14 or 1:12 would do what you want.

Varget was just too slow. Ramshot TAC was getting there, but nothing like the AA1680.

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Abbreviations used in Reloading
Pointed Soft Point "Core Lokt"

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