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Stability with Marlin 444 Microgroove 1-38" twist

Posted: Sat Nov 28, 2015 11:37 am
by mtngun
Here's what the Miller Stability Rule says about heavy bullets in the slow twist Marlin Microgroove barrel. I assumed the Fryxell nose design, wheelweight alloy, standard atmospheric conditions, and 1800 fps.

300 gr. -- 1.43 stability factor (good)
325 gr. -- 1.23 stability factor (ok)
350 gr. -- 1.03 stability factor (risky)
375 gr. -- 1.00 stability factor (not good)

Less than 1.0 is considered unstable.
1.0 to 1.2 is considered marginally stable.
1.2 to 1.3 is popular for 100/200 yard jacketed benchrest shooting.
1.5+ is popular for hunting rifles.

Conclusion: the Marlin 1:38 twist should be OK up to 325 grains with wheelweight alloy. This is consistent with real world experience.

Re: Stability with Marlin 444 Microgroove 1-38" twist

Posted: Sat Jan 16, 2016 12:19 am
by 6pt-sika
Well lets see over the course of the last eight or so years I've had you cut me a 375 grain mold , had LBT cut me one and had BRP cut me one in a FN and HP version . I've also had you cut me a 365 grainer . I have a couple versions of the Ranch Dog 350 in FN and HP as well as an LBT in 340 grain .

All of these were cut for and have been used in any number of the 24 or so Marlin 444's I had with 1-38" twist . And as long as I had a decent scope on the gun , water quenched the WW alloy and pushed them a bit I got acceptable hunting accuracy .

So I can honestly say I do not agree with your table .

I might add all but the BRP/RD 432-375GCHP and the MM/RD 432-365GC have been used to kill deer with very desirable results . And once a deer walks in front of me when I am using these two bullets I certainly expect them to have excellent results as well .

Re: Stability with Marlin 444 Microgroove 1-38" twist

Posted: Sat Jan 16, 2016 12:39 am
by 6pt-sika
Couple years ago I did some side by side velocity checks with the same bullet/loads in my 1-38 24" and 19" guns .

I got the following averages on the same day .

Ranch Dog 432-350GC with 48 grains of H322 .

19" barrel 1975 FPS avg for six shots

24" barrel 2055 FPS avg for six shots

BRP/RD 432-375GC with 47 grains of H322

19" barrel 1923 FPS avg for six shots

24" barrel 2006 FPS avg for six shots

Both of these bullets were wheel weight alloy that was water dropped from the mold . I did find in a 1-38 micro barrel I needed to water quench anything over about 315 grains .

Re: Stability with Marlin 444 Microgroove 1-38" twist

Posted: Sat Jan 16, 2016 10:36 am
by mtngun
Thanks for your input. Real life data always trumps theory. :)

That said, I remember you posting a target with holes that looked to me like the bullets were tipping a wee bit, though you thought they looked fine.

Temperature, humidity, and velocity have a big impact on stability, as does any bullet deformation or casting flaw. Hence the general rule of thumb is to avoid operating on the ragged edge of stability. This is especially true from the mold maker's perspective if his customers expect his recommendations to work in many different loads and conditions.

Your loads are running at 1900 - 2000 fps while my recommendations for the 444 deliberately chose a more conservative 1800 fps, which reduces stability because the bullet does not spin as fast. Bottom line is that the Miller formula is just an estimate, intended to put us in the ballpark. It seems to be more accurate than the old Greenhill formula but it's still just an estimate.