Gain twist and cast bullet mathification?

Elric

Elric

Well-Known Member
After another night without alcohol to power cogitation (Bender is my hero), I came across the article by Harry M. Pope on his barrels. He used a gain twist. And a taper from breech to muzzle.

Has anyone delved into the mathification of calculating the relationship between alloy, bhn, rifling, pressure, and velocity?

I don't have anything to do with gain twist. No inclination, either...

PS. So, Pope made muzzle loading (using false muzzles), and breech loading barrels. The 200 yard group guarantees he gave in his article were 3 inch for muzzle loading rifles, 2 1/2 for breech loaded rifles. Methinks the slightly larger muzzle loaded groups are due to the taper in the barrel. Larger at the breech, smallest at the muzzle. So if you load the bullet from the muzzle, it will be swaged down to fit the smaller end, and when it reaches the (relatively) wider breech end, it will be a wee bit smaller. But... if the powder is quick, it should smack the base and upset it...

Breech loading would start the bullet off at the wider end, and as the bore tapers, the bullet will "squeeze" down.

Maybe that is why, for tapered bores, the ML is slightly less accurate. For a straight bore, I'd say a ML bullet would be just as accurate as breech loaded...
 
Elric

Elric

Well-Known Member
Bartlein Barrels

https://bartleinbarrels.com/barrel-faq/

What are the benefits of the gain twist type rifling?

" I’ll quote what Pope (Pope was one of the greatest barrel makers from a bygone era. His barrels along with Schalk who he learned from and gives credit to and Schoyen, and Zischang made barrels for the Schutzenfest type of guns/shooting in the late 1800’s to early 1900’s) said around a 100 years ago first. “The advantages of the gain twists are three. 1st The twist being less at the breech, gives less friction to the bullet; it therefore starts easier and quicker, giving the powder less time to burn on in front of the chamber, which therefore fouls less than in a barrel of uniform twist at the same necessary muzzle pitch (twist). 2nd The slight change in angle of the rifling, in connection with choke bore (lapping choke bore of the barrel), effectually shuts off any gas escape of gas and prevents gas cutting, which is another case of imperfect delivery. 3rd It holds a muzzle loaded bullet in position much better than a uniform twist….

Now I will add some more to this. First off I feel this applies more to a lead bullet shooter than a jacketed bullet shooter but some of the why’s and why not’s do overlap. With a gain twist barrel the bullet cannot go to sleep. The rifling is always putting a fresh bite on the bullet as it goes down the bore of the barrel. This is why I always go back to a cut barrel being better than a button barrel. A cut barrel even with a straight twist is more uniform and consistent than a button barrel. With button rifling the button can hit a hard spot/soft spot in the steel and it will slow the button down. The button could speed back up and do the twist it’s suppose to be doing but either way you end up with a non uniform twist and it the twist keeps getting slower towards the muzzle. These two things are a accuracy killers and lead to consistency problems/fliers etc… I feel even a slight gain twist will help accuracy wise and not hurt a jacketed bullet shooter as well. For the most part I would say there is no velocity gain in a gain twist barrel with the same load. What has been conveyed to us and it goes back to Popes 1st point is that shooters have noticed that they can run a slightly heavier powder charge vs. a shooter with a straight twist barrel. As the bullet is starting easier into the rifling my only guess is the pressure isn’t spiking as fast or is delaying the pressure curve. Hence forth they can get more velocity out of the gain twist barrel. I feel pressure is pressure and that the twist doesn’t have anything to do with pressure for the most part but my only guess is that the gain twist like I said earlier is delaying the pressure curve. So you don’t see problems as early like hard bolt lift etc… Also it’s noted that even now a days our military in some 20mm and the 30mm barrels like on the A10 Warthog ground attack aircraft have gain twist type rifling in the barrels."
 
Elric

Elric

Well-Known Member
http://benchrest.com/showthread.php...ls-Much-ado-about-nothing&p=543790#post543790

"What is not often discussed are several facts about gain-twist barrels and their effects on the bullet itself: the grooves change width as well as pitch. In a cut-rifled barrel, the grooves decrease in width as the pitch increases, because the aspect of the face of the cutter changes as the angle of attack increases: this would seem to be preferable in a mechanical sense because the changes in the engraved and un-engraved surfaces of the bullet (grooves become wider, while the 'lands' on the bullet body increase in width) help to preclude escape of gas past the bullet. In a button-rifled barrel, the grooves must necessarily increase in width as the pitch is increased, because, unlike the single-point rifling cutter, which has an essentially perfectly sharp crest and clearance behind the edge, the button has lands of finite length in order to push-up a standing wave of barrel material, then ride over it and smooth it in passing, and the only way the pitch can be increased is by driving the button at a pitch faster than the basic one engraved in the button's body - the result is that the barrel grooves increase in width and the lands become narrower, which seems less desireable than the case with cut-rifling. Then, too, consideration of the forces on the button in rifling with a fixed-pitch make one wonder how good the results can be from trying to drive the button at an ever-increasing pitch as it is forced through the barrel: button-rifled barrels already suffer from pitch variations due to such factors as varying hardness of the barrel steel and the lubrication of the bore, and attempting to force it to rotate faster than its basic pitch, on the end of a long rod of less-than-bore diameter (which can be deflected measureably with no more force than the hands can apply), is a very uncertain process."
 
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