TargetTalk

Sorry if this is a bit techy, but I'm interested in the rifling arrangement.

From what I've seen, rifling is like a thread with a constant rate of rotation throughout its length - please correct me if I'm wrong!

So a bullet entering the throat is immediately met with a section of rifling which will eventually dictate the spin rate of the bullet - but the bullet has to accelerate angularly to achieve that spin rate, it absolutely cannot do it instantaneously.

So I'm imagining significant deformation of the bullet where it contacts this initial rifling, possibly even shearing off of slices of bullet material in the first several inches of the bullet in the barrel, until the thing is stabilised in the commanded spin rate, in the groove, as it were.

Given the apparent care with which we like to treat the throat regarding cleaning, bore guides etc, are we not kinda wasting our time? The bullet is doing much more to the throat than a length of cleaning rod could ever do. Surely?

Waisted,

You are stating the problem as though the bullet acclerates to its final velocity and then hits the rifling to pick up the spin. Both happen at the same time.

Most here are shooting some form of bolt action rifle. When the round is chambered and bolt closed the bullet is forced into the throat of the chamber. At this point the lands of the barrel have cut into the sides of the bullet. velocity=0 angular velocity =0.

When the round is fired the bullet accelerates from 0 on both along the direction of travel and rotationally. While this happens very quickly (on the order of a millisecond) it is not instantaneous. Energy from the burning powder is converted into both linear and angular kinetic energy.

If you ever recover a slug that has been fired into water or gelatine you will see no shredding of lead involved just some nicely impressed marks where the lands cut into the slug.

The numbers are impressive. Assume a twist rate of 1 every 12 inches (1 twist per foot) Next the bullet supposedly does most of its accelerating in the first 12 to 14 inches of barrel (this is a std vel 22) For round numbers I will say it hits 1000 feet/sec in 1 milisecond. That is an acceleration rate of 1,000,000 feet per second per second. Fortunately it only does this for .001 seconds. Now 1 twist per foot, that means 1000 revolutions per second or 60,000 revolutions per minute. The numbers are pretty impressive when you work them out.

While even 22 barrels do eventually wear out, lead by itself is much softer than the ordinance steels that go into the barrels so the lead takes the abuse. Also, lead is a pretty decent lubricant against steel.

Cheers 'Dude

There are still folks fooling around with gain-twist rifling, but changing the pitch of the rifling REQUIRES shredding the jacket/outer layer of the bullet. With lead bullets one might expect heavy leading. Pretty impressive accuracy has been achieved with constant twist. Are there and BR competitors using gain-twist?

Thanks folks! Once it has been explained about the zero linear and angular velocities, it's freaking obvious! Makes my post a bit dumb, sorry.

<runs off looking for a head-sized paper bag>

Good explanations in the above posts :-)

OTOH there's Swedish PCP air rifle makers FX that have recently started marketing "Soft Twist" barrels. These are smooth bore with rifling only at the muzzle end. So here the pellet builds up speed before hitting the rifling. According to forum reports I've seen, they achieve very good accuracy.

Waisted, don't feel bad about your post - you're right, in many circumstances.

Think of a revolver. The projectile has to fly forwards a considerable distance before it encounters any rifling. This is even worse for lead wadcutters, completely buried in the case. Also, the extent to which a chambered round engages the rifling varies from one type of gun to another. Have a look - load a round, then remove it, unfired.

F W Mann, in the years around 1900, did a lot of work on how lead projectiles were deformed ('upset') into the rifling. His conclusion was that the lead projectiles behaved rather like putty. His experiments were with full-bore rifle rounds, but I'm sure the principles he wrote up apply (to some extent) to .22 ammunition.

Another related point. If the bullet is following the rifling, it becomes increasingly important to keep the lands and grooves clean so it can follow them! I listened to one group of friends who swore no barrel cleaning is needed with waxed/lead bullets. Another group of friends swore that barrels should be cleaned after every outing. Guess what - my accuracy increased a noticable amount.

Regarding my previous post:
the correct name seems to be "smooth twist" (ST) barrel in case you want to Google this term.
It also seems there is some worry among users about the rifling shedding lead off pellets because they build up speed before hitting the rifling and starting to rotate.
Some FX models seem to be offered with a choice of ST or LW barrel.
I guess FX has created a practical testbed for the ideas discussed in this thread :-)