Progressive twist rifling?

[Rebar]

Looking through the different surplus C&R rifles, I came across the Italian Carcano. What struck me about it, it has "progressive twist" rifling.

Thinking on it, it seems like a pretty good idea. I'm wondering why that current military rifles, and machine guns for that matter, aren't being made with progressive rifling? If the Italians could do it 100+ years ago, it must be well within a modern factory's capability.

For example, the current m16 has a 1:7 twist. The inertia of the bullet as it travels down the barrel, it must be sliding past the rifling before it spins up to the correct rate. What if the barrel was, say 1:14 which progresses to 1:7?

Or is the effect too minor for the extra expense, or is there extra expense? It would seem that even a minor increase in barrel life could be worthwhile, especially in machinegun barrels.

 

[HankL]

Do a google search of "gain twist" plenty of reading on the subject.
Have fun.

 

[Rebar]

Hmm... didn't find much pertinant info with that google search, except that it's quite popular for large naval guns!

 

[30Cal]

From looking at spent slugs picked up from downrange, I don't see too much evidence that the bullets are "slipping" (except in the case of soft or overly driven lead bullets where leading is evident in the throat).

 

[Jim Watson]

Colt percussion revolvers and Pope target rifles had gain twist rifling.
It comes up every once in a while; there was a Gaintwist Barrel Co. just a few years ago; but does not catch on in modern calibres because it cannot demonstrate an advantage worth the extra trouble and expense.

 

[Rebar]

it cannot demonstrate an advantage worth the extra trouble and expense.

From what I understand, most mass-produced barrels are hammer forged over a rod that has the "negative" rifling on it.

Seems to me it would be very little trouble to make the rod with progressive rifling. Or am I misunderstanding the process?

 

[Black Snowman]

Forging in progressive rifling in isn't an option because you then cannot remove the rod with out damaging the rod or the rifling as it's not a constant twist it's locked in place. So the rifling has to be machine cut.

TheSchuemann AET barrels use progressive rifling as do some paintball gun barrels.

 

[Rebar]

it's locked in place.

Yes, I can see how that'd be a problem.

 

[Sleeping Dog]

Why bother with progressive rifling? For the first few inches, the bullet isn't moving fast enough to shave metal into the grooves. By the time the bullet is moving fast, it's also spinning fast.

So even with constant rifling, the spin rate is progressive.

(I think)

 

[Ivy Mike]

The bullet leaving a constant twist barrel should be rotating at exactly the rate of twist. (at least, it sounds like it) A .40S&W should be spinning once every 16" No more, no less. The bullet may not engage a constant twist right away, but by the time it leaves the bore, it is spinning that fast.

The progressive rifling just sounds like a way to get larger, heavy bullets to spin without ruining the bore of a big expensive gun. It may help a modern firearm, but bullet technology is so good anyway, it doesn't sound necesary.

I am just hypothesizing as I know very little about how bullets actually behave in barrels.

 

[schizrade]

I am just hypothesizing as I know very little about how bullets actually behave in barrels.

Good hypothesizing!

 

[Gordon]

Gain twist doesn't work well. While Pope may have fooled with it all the Pope barrels I've seen and owned had nice deep cut rifling of conventional twist. I think a slightly 'choked' barrel can be real accurate but are a real pain to produce. I've had a few gain twist guns over the years, me liking the oddball stuff and all, and none of them showed very good accuracy.