Progressive Depth Rifling

[Southron Sr.]

Interestingly enough, "Progressive Depth Rifling" was one of those "accidental discoveries" that was brought about when the French were rifling some of their old smoothbore muskets so they could be used with the new French invention: the Minie Ball.

As the barrel metal at the muzzle of the smoothbores was so thin, there was no way full depth rifling could be used without dangerously weakening the last 6 or so inches of the barrel. The solution: Make the rifling grooves deep in the breech where the barrel walls are THICK and then make the rifling grooves VERY SHALLOW towards the end of the barrel, so the thin muzzle area of the barrel would not would be weakened.

What the French found was that those muskets shot BETTER than arms rifled with rifling grooves that were the same depth throughout the length of the bore.

ANOTHER ADVANTAGE OF "PROGRESSIVE DEPTH RIFLING?"

I have a friend with a replacement Springfield barrel that has a .580 bore [measured with a plug gauge] but the rifling is 1 in 72" and IS NOT "Progressive Depth Rifling."

My son has an original '64 Springfield with a barrel that has "Progressive Depth Rifling." It too, has a .580 bore [measured by plug gauge.]

The difference:

Load my friend's Springfield with Minie Balls sized .575 and about half the time, they will Keyhole in the target at 50 yards.

Load the same bullets in my son's '64 Springfield and they will never keyhole and shoot to the "Point of Aim" even though they are 5 Thou UNDER bore diameter! In other words they are accurate.

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Original P-53 Enfields had Progressive Depth Rifling. The service cartridge adopted with the Enfield was a paper patched, smooth sided bullet of .568 diameter that utilized a plug in the hollow cavity to aid expansion. Due to complaints of soldiers in the field about their Enfields being hard to load after a few rounds, the British army adopted a .550 diameter bullet that was basically identical to the .568 bullet EXCEPT for diameter.

What they found was that the .550 bullets WERE MORE ACCURATE than the .568 bullets!!!!
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POINT TO PONDER: Maybe the old Skirmisher's Tale that for BEST accuracy, only bullets 1 or 2 Thou under bore diameter will produce the best accuracy DOES NOT NECESSARILY HOLD TRUE for arms with Progressive Depth Rifling [But does hold true for rifles with rifling of uniform groove depth throughout the entire barrel.

Arms with Progressive Depth Rifling can shoot under size bullets with amazing accuracy.

WHAT IS YOUR OPINION?

Will Progressive Depth Rifling work with undersize bullets and still produce exceptional accuracy?

THANKS FOR YOUR REPLYS IN ADVANCE !

Please share your opinion with us!

[DAVE FRANCE]

Hello Southron,
I looked in a book UNDERSTANDING BALLISTICS. It says the US did use progressive rifling for 11 years, based on testing the French did. It doesn’t say why the US stopped using it. The book says the US groove depth was 0.015 inches at the breach and 0.005 at the muzzle of muskets. Maybe the US found it cheaper to manufacture barrels that had deep rifling from the breach to the muzzle.

How deep is the rifling of US muskets made with normal rifling?

In Mann's book he showed pictures of cast bullets that were shot from a very short barrel (1 or 2 inches). The rear part of the bullets were mushroom shaped.

I expect that bullets (especially Minies) will expand enough to fill deep rifling. If the bullet expands well and doesn’t cant too much in the bore, it ought to shoot well with progressive rifling. Even if the front part of the Minie is under bore size, it ought to expand in the front up to the bore size well enough to shoot well.

David

[DAVE FRANCE]

Hi Southron,

I have a copy of a report about developing the ammunition, twist, rifling, etc. for the 1855 rifle. Written by US Army Ordnance.

It mentions the decision to change to 58 caliber and the change to progressive rifling (0.015 to 0.005 inches) and that progressive rifling gave better accuracy. It doesn't say what the clearance was between the bullet and bore in the tests.It also says the variation in bore will be changed from 0.010 inches to 0.0025 inches, which probably means there may not have been a need for progressive rifling because the the maximum difference between the bore and the bullet outside diameter was close enough for the normal rifling to work.

I didn't read it carefully and I may have missed something. I don't usually keep things like this; if you want it I'll send it to you.

David

[Blair]

Hi all,

My understanding of "progressive" rifling is that the depth only effected the groves (the max bore dia.) and not the lands (the minimum bore dia.)
The difference between these two depths, at the muzzle compared to the breech was only about .001 of an inch per grove (A total difference of perhaps .002).
This would allow the skirt of the bullet to expand into the groves at the point where the pressure was greatest during ignition (the chamber). The bullet would then be able to maintain a good gas seal, in effect being swaged, down the length of the barrel due to the slight restriction of the ever so slightly decreasing grove depth.
Blair

[Southron Sr.]

Dear Dave & Blair:

My understanding has always been that "Progressive Depth Rifling" refers to the fact that the grooves are deeper in the breech than at the muzzle. The lands always remain the same.

What originally brought my post about was the stark difference in accuracy between two rifle muskets shooting undersize bullets. The rifle-musket with the Progressive Depth Rifling shot amazingly well compared to the one that did not have PDR.

My understanding, gained from comments made by Space Cowboy on this BB, is that the Enfields produced by Pedersoli WILL NOT HAVE PDR but will, instead, have precision barrels that are lapped. This, of course, means that Pedersoli Enfields will require bullets that are 1 or 2 Thou under bore size for best accuracy.

[Blair]

Again, this is based solely on my understanding of what Pedersoli is going to do with their new Enfield line.
Space Cowboy is more than welcome to correct me.

P-H produced a three band Enfield with progressive depth Rifling with a 1-78 twist. This is what the originals had in them. I can not speak for the two band Naval Rifle.
My P-H Naval only has three L & G's , and I doubt it has a faster rate of twist than 1-66. But, I could be wrong... it was so poorly maintained when I got it from a reenacter, there is simply no way to tell now!
My understanding is that Pedersoli plans on doing the same thing as the original Enfield line had in them. (and in their Springfield line as well)
The idea behind lapping the bore is only to increase their potential for uniformity in accuracy.
Lapped bores maybe an added cost... I honestly don't know yet. But, will pass it on when and if I can.
Blair

[Southron Sr.]

Dear Blair:

From what I understand from reading postings on the various M/L websites:

Parker-Hale produced some of their replica P-53, three banders, with PDR, 3 or 5 Grooves (?) and 1 in 48" Twist for EXPORT to the United States. The Reason? Because Muzzle Loading Association of Great Britain rules FORBID using replica arms that do not have the same type and twist of rifling as the originals.

After the 1 in 48" Twist P-53's were sold, then Parker-Hale also exported P-53's with the regular PDR, 3 Groove and the 1 in 72" Twist. So, depending when a P-H P-53 Enfield was imported into the U.S. the rifling twist can vary.

P-H Naval Rifles have always had the 5 Groove, PDR and with the 1 in 48" Twist. Same with the Artillery Carbines.

Now, to further simplify things:

The Armi Sport Enfield Naval Rifle has the 1 in 48" Twist but only 3 Grooves. I think that it is the Euroarms Naval Rifle that has the 1 in 66" Twist Rifling.

Confusing, Isn't It!

[DAVE FRANCE]

Dear Mr. Sr.

That report I mentioned said that three grooves worked better.

David

[Blair]

David & Brannen,

I believe you are correct. But also, If I am not mistaken, the bullet design is based on the American (James Burton) designed Minnie ball and the development of the Rifle/Rifle-Musket here in the US, and the progressive depth rifling is still used. (If I am not mistaken, it was Burton that got the Brits to reduce the size of the bullets they had been using from a .562 to the .550 dia.)

The Brits used iron and/or bass wood plugs in the base of their bullet to assist in the expansion of the base, along with paper patched bullets to help create a gas seal.
Neither of these features are legal within the N-SSA today. I believe I understand the reasoning for this.

How much this effects what the Brits did with their rates of twist and or other rifling features... I can not say.

I was never aware of the variety of rifling or the variations in the rate of twist produced by P-H, whether for the American or the European market.
That is just based on my own experience,
Blair

[le piaf]

perhaps an help
http://www.researchpress.co.uk/parkerhale/rifles.htm
The hollow base bullet is a design of CaPtain Delvigne (also designer of revolver MAS Mle 1873) , Captain Minié only improved (?) the bullet with a wood slug , but today when we speek of hollow base bullet we said "Minié bullet".

[Space Cowboy]

Southron,

I made the same test with my original Bridesburg rifle musket with progressive depth rifling, and found out the same: the progressive depth helps the bullet stabilize better is it is heavily undersized. I had good results with .581 bullets in the .582 bore, but the rifle worked well with .575 Miniés, even with .570 as well.

The constand depth rifling can achieve the same accuracy, but:
1st: you need a bullet the fits exactly the bore
2nd: if the bore is not polished adequately (both lands and grooves) will loose accuracy because of the fouling.

Pedersoli is not going to reproduce the progressive depth rifling for the Enfields, as it would make the production much more expensive. This is the only reason.

Best regards,
SC