.32-40 & stabilization in a 1:16 twist 26" Marlin

[SierraWhiskeyMC]

Howdy Folks,

Been readin' till eyes bleedin', and still haven't quite been able to determine what the max bullet weight is that will assure stabilization when fired from a 1:16 twist 26" barrel.

Dad left behind Grandpa's mid-20's vintage 1893 Marlin .32-40 when he took his final flight. Along with the Marlin, he left a number of molds, tools, some cast boolits he'd made in the 70's, along with a baker's dozen boxes of REM-UMC and Peters cartridges that are pre-WWII vintage.

After searching through things and trying to figure out what went with what, last night I decided I might as well fire up the pot (a Lee 4-lb bottom pour), toss in some WW's and a bit of Linotype and start making some boolits.

The mold I used is a Lyman 319295 - listed as a 174g GC flat-nosed bullet in the catalog. It's one of the really old molds (pre-1926) with permanent handles. The boolit cavity itself looked very similar to the mold at the end of the Ideal .32-40 reloading tool he'd also left behind.

I cranked out 160 of these, (and a like number of 358477's for .38/.357/9mm while I was at it), and earlier today I seated the gas checks on 'em.

Then I got to thinking; "I probably really ought to weigh these things..."
I sampled four of the boolits Dad had cast, GC'd, lubed and sized sometime in the late 60's or early 70's. I was more than a bit surprised to find that they averaged nearly 186.3 grains! :shock: The boolits I cast from the same mold weighed an average of 178 gr, which I attribute to being cast from ww/Linotype mix rather than mostly lead.

Also, I compared the Lyman 319295 cast boolits with the original Ideal mold on the reloading tool; it appears that the 319295 mold is basically the same mold with the addition of the gas check area; the resulting bullet is longer than the Ideal mold by approximately the height of the gas check. The Ideal mold seems slightly larger in diameter, but I'll have to cast some boolits and take measurements to be certain.

Sure, I could just use the Ideal mold on the reloading tool to cast boolits in the vicinity of 165-170 gr, but then I'd lose the GC

Now I'm thinking that I might be better off using some 323470 165gr GC RN boolits, since he left a mold for those, too .... might be OK if I sized 'em and truncated the round nose, as I don't want any unauthorized primer poking to occur.

As I mentioned already, the cartridge cases I have to work with are quite old, and don't want to do a whole bunch of fiddling.

So, to get back to my original question - anyone know what the maximum weight is in such a barrel for good stabilization?

[tommag]

You need to measure the length of the boolit and use the greenhill formula to come up with an approximate twist rate for stabilization. You can google it or try this one.





http://www.z-hat.com/twistrate.htm

[Bent Ramrod]

I've shot the 319247, the 319295 and the 319162 (nominal weight 185 gr) out of a couple of standard 16" twist .32-40's with no trouble. I used the usual mild loads of 4227, 4198 and SR-4759.

The criticality comes with the target boolits that approach 200 gr. You should be OK.

[beagle]

I've shot the following Lyman numbers in my 1-16 twist 32-40 with no problems.....

319162
319273
32170FN
321297HP
319295

/beagle

[Leftoverdj]

I'd ditch the cases. They are old enough to have been fired with mercuric primers, and have age hardened even if they weren't.

[SierraWhiskeyMC]

You need to measure the length of the boolit and use the greenhill formula to come up with an approximate twist rate for stabilization. You can google it or try this one.

http://www.z-hat.com/twistrate.htm

Thanks - interesting, but it had no input for weight. Which lead (:)) me to continue searching for a more definitive answer.

Greenhill's original formula is too darn simple. It's apparently pretty good if you're using pure cast lead, but not very accurate if you have something else in the mix.

My cast boolits measure 0.95" with gas checks installed.
My Dad's cast boolits measure 0.97" with gas checks installed.

The programs' defaults tell me that my 0.95" boolits will be marginally stabilized with a 1:16" twist, while my Dad's boolits (which are considerably heavier) at 0.97" won't be stable.

Thanks everyone for your comments. Unfortunately, you didn't post what your boolit recipe (lead, tin, antimony etc) content was, so it will be exceedingly difficult to replicate your results.

This is a lot tougher nut than I originally thought to crack.

[felix]

No, no. no! The heavier a projectile is for its length, the LESS twist is needed. ... felix

[Buckshot]

No, no. no! The heavier a projectile is for its length, the LESS twist is needed. ... felix

.............Whut? Er ......................

.............My scheutzen rifle in 32-40 has a 16" twist and it stabilizes the 200 gr Saeco #732 at 200 meters just fine.

http://www.fototime.com/A8D46B856BF255E/standard.jpg

In case you're wondering, it will NOT stabilze these 8mm 216 and 236 gr slugs :-). However a good choice for your rifle would probably be the Lyman 8mm 323470, if you bump a FN on it. It's a 165gr RN (from the mould) and takes a GC.

..............Buckshot

[SierraWhiskeyMC]

No, no. no! The heavier a projectile is for its length, the LESS twist is needed. ... felix

Right - I wrote it backwards. :oops:

Buckshot wrote:

However a good choice for your rifle would probably be the Lyman 8mm 323470, if you bump a FN on it. It's a 165gr RN (from the mould) and takes a GC.

I have that mold; mentioned it in my 1st post (3rd from last paragraph). I think I'll have to crimp them below the top driving band rather than the crimp groove to get the OAL near correct - after truncating the RN.

But after thinking about the history of this particular 1893 Marlin for over a month, I finally remembered that sometime around 1970 my Dad had a live round misfeed aka "the dreaded Marlin jam" - and he had a heck of a time getting that live cartridge out.

I hadn't planned on making any modifications to the rifle, but I guess I'm going to have to take it down and see if this mod is necessary:
http://marauder.homestead.com/files/Marlin94Fix.html

[SierraWhiskeyMC]

After more looking around, I found this page:
http://accurateshooter.wordpress.com/2008/06/03/calculating-bullet-rpm-spin-rates-and-stability/

There's a downloadable Microsoft Excel spreadsheet that incorporates Don Miller's stability formula. Not sure if it'll work from here, but the direct link to the spreadsheet is:
http://accurateshooter.net/Blog/millerformula.xls
His formula calculates stability by caliber (in inches), bullet weight (grains), bullet length (inches), barrel twist (inches per turn), muzzle velocity (FPS), temperature (°F) and atmospheric pressure (inches of mercury). You'll note that a bullet which is marginally stable at 80° may not be stable down near freezing; something to keep in mind if you're working up loads this summer for deer season this fall.

In case you don't have Microsoft Excel, the formula is:
Sg=(30*BulletWtGr)/((BarrelTwistInPerTurn/Caliber)^2*Caliber^3*BulletLenIn/Caliber* (1+(BulletLenIn/Caliber)^2))*(MVfps/2800)^(1/3)*((TempF+460)/(59+460)*29.92/AirPressInHg)
If Sg is less than 1.4, bullet won't be stable.
If Sg is greater than about 2.0, you might gain accuracy by going to a slower twist barrel.

[beagle]

Don't ditch them, aneal them. I'm still using old Peters cases in my .32-40. Neck aneal and they still keep on kicking./beagle


I'd ditch the cases. They are old enough to have been fired with mercuric primers, and have age hardened even if they weren't.

[SierraWhiskeyMC]

Don't ditch them, aneal them. I'm still using old Peters cases in my .32-40. Neck aneal and they still keep on kicking./beagle

Annealing certainly is much more attractive than just ditching these 230 some cartridges; particularly since Midway won't have any more cases in stock until either the end of the month or more likely next month - and it would run me around $130 to replace 'em, when I'm probably not going to be doing a whole lot of shooting with it in the near future anyway.

[Frank46]

Well I could be full of horse pucky so here goes. My lyman tong tool mold cast a 165 grain plain based bullet. So We can say that basically from 165grs to about 200 graing would be the limit for the 1x16 twist. But some shooters have discovered that thrie bullets tip going through the target. That to me is instability due to the twist not being fast enough. What then are the choises? way I see it there are two.
Drive the bullet faster or go from 1x16 to 1x15 where the instability disappears. But this is not a hard and fast rule. Some 1x16 twisted bbls will shoot 200grain boolits very well and some won't> I'm no ballistician by any means but can draw some conclusions from the bench rest shooters. Alot of their shooting is done with barrels on the ragged edge of stability. And they get great scores. here they CAN and will drive their bullets way faster than you or I to overcome the instability issue. With our alloy bullets driving them faster won't always work due to pressure and velocity limitations. And in certain cases our lead alloy bullets at certain velocities are much more affected by wind. Since their bullets are by their very nature are harder they are being shot faster to comphensate for wind changes. Something to ponder on. Frank

[SierraWhiskeyMC]

Well I could be full of horse pucky so here goes.
My Lyman tong tool mold casts a 165 grain plain based bullet.
Which mold do you have? 319247? A number of people have said that is close to, or is, the original design for the .32-40 boolits.


So We can say that basically from 165grs to about 200 graing would be the limit for the 1x16 twist.
There's more to it than just the weight of the boolit and the twist of the rifling. The caliber and length of the boolit, muzzle velocity, air temp & pressure all need to be considered.

Changing the alloy used from nearly pure lead to Lyman #2 alloy, or perhaps Linotype could make a reasonably stable bullet unstable, simply due to the change in boolit weight.

A boolit that's stable when it's 80° outside might become unstable and tumble at lower temperatures. This actually happened to some US military service rifles when they tested them in cold regions; I don't recall for certain which rifles were affected, but I think it was the M16A2 when they increased the projectile weight. They wound up increasing the twist rate.


But some shooters have discovered that their bullets tip going through the target. (you're talking keyholing there.) That to me is instability due to the twist not being fast enough. What then are the choices? Way I see it there are two.
Drive the bullet faster or go from 1x16 to 1x15 where the instability disappears.
Well, I wouldn't even consider changing out the barrel on my 1893 Marlin, as it's around 85 years old and still original.

A higher MV would help to stabilize the boolit, but I don't think I'd want to push the MV beyond 1800FPS or so; too much stress for an old bangstick. I'm going to work with some loads in H4895 and Varget to try to get the MV up, but keep the breech pressure down - while keeping the rifle clean, too. I think somewhere between 21-25 gr of H4895 behind a 165gr GC boolit will get in the vicinity of 1800FPS in that 26" barrel, but I haven't started loading anything yet. I don't think I'll try to push the 178gr boolits quite that fast. The goals here are to have fun, work up an accurate load or two & not break anything.


But this is not a hard and fast rule. Some 1x16 twisted bbls will shoot 200grain boolits very well and some won't. I'm no ballistician by any means but can draw some conclusions from the bench rest shooters. A lot of their shooting is done with barrels on the ragged edge of stability. And they get great scores. here they CAN and will drive their bullets way faster than you or I to overcome the instability issue. With our alloy bullets driving them faster won't always work due to pressure and velocity limitations. And in certain cases our lead alloy bullets at certain velocities are much more affected by wind. Since their bullets are by their very nature are harder they are being shot faster to compensate for wind changes. Something to ponder on. Frank

Well, once you start adding other metals to lead, you're likely going to wind up with a boolit that occupies the same amount of space, but weighs less; therefore crosswinds will push it around more.

But back to the stability issue; there are a number of things that can be done to increase it:
1) Use a heavier casting alloy.
2) Use a shorter length boolit mold.
3) Increase muzzle velocity.
4) Shoot on warmer days.
5) Shoot on more humid days.
6) Shoot at higher altitudes.
7) Increase rifling twist rate.
Humidity isn't figured into Don Miller's formula, but it's a well known fact (particularly to aircraft pilots) that humid air is less dense than dry air. (I know, that sounds completely backwards - but it's a fact.)

[HORNET]

Those 186 gr. 319295's should stabilize just fine as will the 319297. I could almost get them stabilized in a .32-40 I had with a 20" twist (re-chambered Martini Cadet) but didn't want to push a thin barrel very hard.