Question about boolit jump.........

[Knarley]

The question I have is............. what does touching the lands do that is beneficial? ( Don't tell me it shoots better)
One would think that it would build up pressure before the projectile moves causing a pressure spike. Or is that is what is intended to help combustion of the powder? I have wondered how far everything moves down the barrel before combustion begins. A shot gun primer will send a wad quite a ways, does touching the lands stop this?
One would think that "getting a run" at the lands would help the projectile on it's way down the barrel, some what like gaining speed to plow through a snow drift.
Against the lands under pressure, I can see the boolit "Bumping Up", slamming into the lands whist moving will do the same, no?
I can't see that having the projectile moving a bit "unguided" by the lands would hurt, it isn't out of the case by the time it engages.....

Yeah, I know, I think too much.............it's a "gift"

Knarley

[Boz330]

Makes it shoot better or maybe not. A boolit that has to jump to the rifling can possibly tip slightly and enter the barrel off center. Probably not an issue at close range but at distance it will be. If the bore rider section of your boolit is a snug fit probably doesn't make any difference. The resistance of the 1st drive band being in the rifling will build pressure and give a cleaner burn.
Best thing to do is try both methods and see which works better. A buddy of mine had a gun that liked being backed off of the rifling several thousandths.

Bob

[dtknowles]

A better question is: "When a load shoots better when touching the lands, why is that?"

It is not always better and it is not always better in the same way. Since this is a black powder thread I think the bumping up is relevant and the bullet tipping a little less so.

I think bullet design and fit plays a lot into this conversation. Some black powder cartridge arms do not have enough space in the neck of the case to fit a bullet of sufficient diameter to fill the grooves so it the bullet can be bumped up to groove diameter as it enters the rifling that is a good thing for those guns. On the bullet tipping, that applies more to longer bullets.

If we were talking about jacketed bullets and smokeless powder bench rest shooting it is a different story but just as interesting.

Tim

[DougGuy]

I can't relate to BPMC guns, but my .308 shoots so so groups with a 180gr j-word crimped into the cannelure, however pushed out forward .100" sets it .025" from the lead in of the rifling, and it cuts groups by 2/3!

By contrast, the .45 ACP barrels I have been throating with .150" or so freebore before the lead in begins shoot lights out! Go figure!

[martinibelgian]

The intention is to have enough initial resistance to stop the bullet moving when that primer lights - the movement can be larger or smaller from shot to shot - variation. By either having sufficient neck tension or the resistance of the bullet seated into the lands, you create enough resistance to stop the bullet moving from primer ignition alone before the main charge lights. That bullet moving forward just a little bit or a bit more will give you a firrent-size combustion chamber to start out with, so pressure and MV variation. So you can use slip-fit with the bullet seated hard into the lands, or neck tension with the bullet seated off the lands - but a slip-fit bullet seated off the lands usually won't cut it accuracy-wise.

[justashooter]

the best accuracy you will ever get with a BPCR is when you use a "false muzzle" to load the bullet from the muzzle end and use cartridges that are loaded with charge and capped by a paper wad. the famous target cartridge 8.15X46R used to be marketed with loaded cases this way in 3 power levels for mid range target shooting. H M Pope won many 200-1000 yard matches using this technique. the advantage was in getting the bullet seated perfectly concentrically and "pre-engraved" for best stability in flight.

seating bullets out to partially engrave or at least touch the lands is the same thinking, to a smaller degree. it is all about concentric stability in flight.

[country gent]

A bullet getting a "running start" on the rifling develops momentum and dosnt start spinning from the beginning once moving straight ( so spin) the bullet may slip or skid at first starting into the rifling with a large jump. STarted close or touching this effect is reduced some. Another is starting into the rifling allows the ogive or driving band to help center bullet with bore better and straighter. Even with a bore riding desighn unless its a snug fit the bullet in to the throat can help center and hold.

[ascast]

firrent-size ???

[Patrick56]

The question I have is............. what does touching the lands do that is beneficial? ( Don't tell me it shoots better)
One would think that it would build up pressure before the projectile moves causing a pressure spike. Or is that is what is intended to help combustion of the powder? I have wondered how far everything moves down the barrel before combustion begins. A shot gun primer will send a wad quite a ways, does touching the lands stop this?
One would think that "getting a run" at the lands would help the projectile on it's way down the barrel, some what like gaining speed to plow through a snow drift.
Against the lands under pressure, I can see the boolit "Bumping Up", slamming into the lands whist moving will do the same, no?
I can't see that having the projectile moving a bit "unguided" by the lands would hurt, it isn't out of the case by the time it engages.....

Yeah, I know, I think too much.............it's a "gift"

Knarley

Seating the bullet in the land does the same as an uniform factory crimp. It has to do with the load density. If the bullet jumps when the pressure is low it causes irregular pressure build up due to the change in chamber volume versus load. The burning rate of powder is not linear. Higher pressure = faster burning rate.

[texasmac]

Keep in mind that when using black powder the fouling can cause a problem if the bullet is seated hard into the leade on a clean chamber. Depending on the rifle action, as the throat becomes fouled from subsequent shots, cartridges may become hard or impossible to fully chamber. Of course that may not be a problem if one wipes between shoots. I finger seat my bore-riding bullets 0.005" off the leade and get excellent accuracy with my Browning .40-65 BPCR using a blow tube.

Wayne