Gain twist rifling

[paulvallandigham]

The increase in speed occurs because of expanding gases. No expansion, no increase. The pressure may be holding and then reducing, but its directly proportion to how far down the barrel has moved, with allowance for escaping gas out vents in flintlocks, or flashchannels in percussion locks. The expanding gase is what causes the crack of the sound barrier when the ball or bullet leaves the muzzle, even when the ball or bullet is traveling at below the speed of sound. The gases are still traveling much faster than the speed of sound.

With smokeless powder, the powder is consumed and maximum about of gas is produced when the bullet is as little as 12-16 inches down the barrel. In longer barrels, there is actually some drag that reduces the velocity of the bullet, but you have to use test barrels to prove it.

 

[shortstring]

OK, for the sake of conversation lets say that the diameter of a bullet IS expanding 10,20 & 30 inches from the breech. As you say this would insure a good seal all the way out & compensate for leads lack of reseliency. Why is it common for slug guns to have choke? There's no reason for the choke if the lead is expanding. For that matter a bore that was reversed choked would still remain sealed if this is true. Why also would this same situation not exist in a gain twist barrel? If it did, the expanding lead & the ever changing pitch of rifling could be the best of all worlds.

I agree that the expanding gases cause the bang. This is how a silencer works, it cools the gases before they reach the open air. But the expanding gas doesn't parallel the pressure in the barrel. If we were to work with 10,000 PSI & have it remain constant as the bullet traveled out of a 30" .50 cal barrel the force as the bullet exited would be 471,240 pounds pushing out on the bore. I personally don't want to be anywhere near an A weight .50 with 471,240 pounds trying to get out.

You also say that a bullet would lose its ability to remain sealed because of the changing pitch in gain twist rifling. How & why then did Pope & others make some of the finest barrels ever with gain? They had to remain sealed.

Paul

 

[paulvallandigham]

The bullets that Pope used with his Gain Twist barrels were paper patched. That is how they maintained that seal.

As to your question about slug guns, are we talking ML rifles, or shotgun slugs? Most slug gun bench rifle ML shooters DO choke their barrels but the choking is measure in the thousandths of an inch, and not like the choke in a shotgun barrel, for instance. The slug gun shooters ( and some chunk gun shooters) choke the barrels to keep resistance on the PRB they fire( chunk gun) or the paper patched bullet they fire from slug guns. The belief is that friction either dries the paper, or that the paper becomes compressed as it moves out of the barrel, so that choke helps the accuracy by putting more pressure on the paper patch as it nears the muzzle.

Now, it certainly would be true that for most guns, a paper patched bullet fired from a barrel with the choke at the breech, and the slightly greater size at the muzzle is not going to adversely affect accuracy very much. But group for group, the average group of that choked bore, with the choke near the muzzle will produce better results.

We don't usually discuss paper patched bullets on this site, and I really don't want to get much deeper into the subject, unless it has to do with a question that better fits in the Muzzle loading Accuracy topic, rather than the Gun Builder's Bench. I have spent a lot of time watching and discussing paper patched bullets with those slug gun shooters- my intention being to see if this is something I could use to improve accuracy in my hunting rifles. I respect the slug gun shooters, and what they do, but have no personal interest in participating in the sport at that level.

 

[CoyoteJoe]

Shortstring, I think you may have misunderstood Paul's post. I don't believe he meant to say that the bullet was expanding as it runs down the barrel, only that it was still being pushed and accelerated by the expanding gas. The lead bullet does expand, or slug up from the force of acceleration, but that happens only as the bullet first begins to move, or even before it moves and does not continue on down the bore, not at least with normal loads of black powder. So, for the first few inches of it's travel the lead bullet is a very tight fit in the bore. Lead being both soft and non-elastic it soon becomes rather loose in the bore. You can demonstrate that for yourself by just pushing a slug through a rifle barrel, it starts very hard but soon pushes very easily. Thus the choked or tapered bore was developed to keep the slug fitting tightly all the way to the muzzle.
While the advantage of gain twist will probably be long debated, there is little debate as to the advantage of a choked bore. It was conclusively proved 150 years ago that, all else being equal, a choked bore will out shoot a straight parallel bore.

 

[shortstring]

You fail to take into consideration that the pressure on the base of the bullet is driving the bullet faster and faster down the barrel, and that will cause a lead bullet to expand outward to fill in any area where lead might be lost to the side of the bore of the barrel.

I don't know? That's what it sounds like to me.

Just for the record Pope lived & shot in the scheutzen era. The bullets were grooved & lubricated. Pope even disigned & sold lubricating pumps for this purpose.

I still like my observation that it's possible that something that made gain twist popular during this era of the elongated lead bullet was that the bullet remained tight in the rifling because of its ever changing pitch. Now, it's also possible that nobody knew that was why it worked. It also would explain why as jacketed bullets became the norm that gain died out. The jacket held its dimension better than lead alloy.

Also I have to admit that I do enjoy this nature of conversation. It seems lately that we set around campfires at shoots & talk about Hillory or the price of gas or who knows. Good stuff.

Paul

 

[Mountainman56]

Wow...thanks to all y'all for your input. Couldn't make up my mind what to do so I ordered a swamped gain twist barrel. Won't get it till next year but I'll let everyone know how it works out once I get it making smoke. Hope it don't turn out to be a wall hanger but what the heck, at least half the fun is in the buildin anyway.

Have fun and be careful.