Variable depth rifling or...

[ryoung14]

...just poor quality?

I bought an unfired Cabela's sporterized Hawken carbine off one of the auction sites recently. It's a .50, with chrome lined bore and 1:24 twist. At the muzzle there's very little depth to the rifling, a few thousandths a most. However, looking down the bore the rifling seems quite deep toward the breech end. The lands stand right up, casting shadows even. Toward the breech there's no problem distinguishing groove from land, while at the muzzle...it's a puzzle :winking: .

Is this something that was done deliberately or was a mistake made while rifling? If deliberate, what's the advantage of shallow rifling at the muzzle? My other barrels have well defined rifling right up to the muzzle; especially the GPR and the GM barrels.

The gun was made by Investarms and appears to be a variation of the Lyman Deerstalker. Hope to shoot it tomorrow.
Bob

 

[Swampman]

Sounds like a coned muzzle, that's not normally found on the less expensive guns. Shoot it and see what happens. :winking:

 

[Toomuch_36]

Sounds like a coned muzzle, that's not normally found on the less expensive guns. Shoot it and see what happens. :winking:

The coned muzzel is being done by more and more makers now, it's purpose is to better align the connicle bullets but not desireable for PRB (at the very least it is of no help).

Toomuch
........
Shoot Flint

 

[Swampman]

The coned muzzle is a huge help when loading PRBs. It lets you throw your short starter away.

 

[ryoung14]

Fired it about 20 times today (all offhand, standing) and it appears to be a good shooter. From the very first shot, if I did my part, they went into the black at 60 yds. When they didn't, I knew why.

A couple of odd things. Using same powder charge, it puts a saboted 300 gr. .44 Hornady XTP at the same point of impact at 60 yds. as a saboted 180 gr. .44 Rem. HP (got a bucketful of those things and the TCs don't like 'em). Also shoots the green Hornady bulk sabots quite well (got alot of them too). Didn't notice any difference in recoil between the 300 gr. XTP and the 180 gr. Rem. . In terms of recoil the little gun's very comfortable to shoot.

One gripe: the unnecessary double-set trigger arrangement puts the trigger too far forward for a comfortable grip. Have to contort my grip a bit to reach the front trigger. Also a heavily gloved index finger won't fit between the trigger and the guard.

It's not a T/C but it's not bad. Probably worth the $180 I spent. Whatever's going on with the rifling isn't hurting anything and it was easy to load. Should make a good tree stand gun.

Oh, yeah, almost forgot. Blew down the barrel after each shot and am still here to tell about it. Even inhaled once :grin:.
FWIW
Bob

 

[Tumblernotch]

Progressive depth rifling was used extensively in rifle muskets during the Civil War. The US Ordnance Dept. did a lot of experimenting with it when switching over to the Burton/Minie bullet.

I don't know how well a PRB performs with this type of rifling, there are probably others here who have experimented with it, but I know that it has been shown to improve hollow base (Minie) bullet ballistics. I would say that it probably improves solid flat base bullets and possibly saboted bullets, although I have no experience with sabots.

With expanding ball (Minies), the skirt expands into the rifling and in travelling down the barrel in a progressive depth bore it gets squeezed tighter, increasing the gas seal and helping it stay in close contact with the rifling. It would probably work very well with a solid flat base bullet as well. You would be surprised at how much a soft lead solid bullet will obdurate (upset) into the rifling when a moderate to heavy charge is used. Like I said before, I don't know what the effect would be on a sabotted bullet. It should work, though it's possible that it could strip away from the bullet if there is a lot of difference in the depth from breech to muzzle. I'll let the sabot experts answer that one.

The Germans used light "squeezebore" cannon, 20 and 30 mm in WW II as anti-tank weapons where the entire bore (lands and rifling) were reduced toward the muzzle quite a bit and they worked. The projectiles used driving bands and/or sabots around a hardened steel or tungsten core.

The Ordnance Dept. also experimented with progressive twist rifling in conjunction with and without the progressive depth rifling. They found that it changed the shape of the bullet a bit too much and the bullet often stripped while going down the barrel. Also it raised the cost of rifling and they pretty much settled on using only the progressive depth rifling.

I'd like to add that I just added the bit about the German anti-tank weapons to show that experiments to improve velocity and accuracy have continued over the years. There weren't very many of these weapons made compared to conventional. The projectiles were expensive and the barrels had a relatively short life due to pressures and friction. If my memory serves me right, experiments have continued along this line in our own Ordnance Dept.

 

[harryo]

The coned muzzel is being done by more and more makers now, it's purpose is to better align the connicle bullets but not desireable for PRB (at the very least it is of no help).

I disagree. I have one barrel that has been coned at the muzzle and it is a joy to load with a patched ball. It makes loading much easier. As Swampman said, you can throw away your short starter, yet still use a fairly snug ball & patch combo and start it with your thumb. I'm condidering having all my roundball barrels coned.

 

[ryoung14]

From the 'coned muzzle' comments it appears that progressive depth rifling means a larger land-to land diameter toward the muzzle (shorter lands). From the Ranger's comment it appears that progressive depth rifling means a smaller groove-to-groove diameter toward the muzzle or a bore where both land-to-land AND groove-to-groove diameters are reduced/'squeezed' toward the muzzle.

Sounds like there's 3 possibilities. What's best for PRB probably isn't the best for sabots/conicals.

I will say that sabots started easily in the little carbine and, as they were rammed down the bore, there was significantly less force needed as they neared the breech. That seems to suggest that the groove dimension is greater at the breech. Whatever it is, it appears to work.
Bob