Learning to make swage dies question.

[lead chucker]

Hi. I have been busy making swage dies. I have zero experience with a lathe up till the last three weeks. I will post pics when I get a chance. Now the question is on a 308 bullet how much lenth of the base of the bullet needs to be .308. Right now I can't seat the bullets very deep in the case and still be just off the lands of the barrel. It took me a lot of polishing to get to .308. And in a blind hole it's hard to focus on one spot to Lap without makeing every thing bigger.

[aaronraad]

Now the question is on a 308 bullet how much lenth of the base of the bullet needs to be .308.

I'll assume you are talking about the projectile shank where the projectile's diameter approximates the nominal calibre? The shank is typically assumed to be parallel, but in practice the shank can taper in both direction, include grooves, driving bands, gas checks, sabot etc.

The shank is designed primarily for accurate alignment and gas sealing purposes within a loaded cartridge and the nominal bore/groove dimensions of a barrel. An under-diameter shank projectile may still get down the bore just as long as it seals enough as it bounces along for pressure to build. At the extreme a projectile may be so under-diameter that it's meplat and base will engage the rifling and form a seal (for a period time) instead of the more ideal shank diameter.

So to increase the alignment of a projectile the length of the shank is increased. The extreme of this is the wadcutter (full-length, not SWC etc.) design which is basically all shank diameter from the base to the meplat. Pistol users would proclaim the accuracy of the design and this meets the needs of short range target shooting (or shooting in a vacuum). The hollow base wadcutter air pellet isn't far behind and is essentially a 'waisted' version of a full wadcutter utilising the alignment properties at the base and the meplat. So there is only a very limited section where the air pellet projectile is not controlled (at least two diameters of contact) in it's mid-section as it exits the barrel.

Most rifle projectiles though have a preference for at least 1 calibre of shank length to maintain accuracy. This nominal 1 calibre length becomes shorter or longer depending on the overall length of projectile. It's also obviously easier to spin a projectile if the shank is centred over the centre of mass for the projectile. Think of where javelin throwers grip and spin their projectile to keep them point forward and impacting the ground at the correct angle.

BR shooters have been known to seat flat base projectiles in 6mm cal with only 0.080" seating depth in case mouth. As to how much neck tension or jump/jam set-up for the rifling used in this situation, I don't know. I do know that with these extremely short seating depths great care has to be taken to assure ammunition is still produced with minimal concentricity and run-out.

BR shooters also talk about a pressure ring, which is +diameter at the base of typically flat base projectiles, or at least very short boat-tail (e.g. 0.060" to 0.080" long) designs. The pressure ring is a very thin section where the projectile diameter might be +0.0003" at the base. From my understanding this pressure ring ideally forms a very positive and repeatable gas seal, which can assist accuracy to forgive variances in match grade barrel manufacturing tolerances, surface finishes and wear rates. Myth or fact, I haven't seen any peer reviewed articles on the subject myself? In relation to bullet seating though this would question if a projectile with a pressure ring on the base is actually having it's neck tension applied across the major shank length, or the just the pressure ring itself. I can understand how soft annealing (repeatable) the case necks in BR can become very critical to accuracy performance though.

Seating depth as a handloading variable overall can be adjusted significantly and still produce accurate reloads without having to be within 0.020" of the lands. Velocity and pressure may be affected as the jump/seating depth is increased and powder charge should be adjusted according to safe levels to account for changes in the case volume. Factory ammunition often has a bad reputation for accuracy with long jumps, especially in target rifles with maximum throat lengths, but that jump is fixed and might easily begin to improve accuracy with +/0.005" adjustment. It's not often though that factory ammunition has projectiles re-seated especially given some of the seating methods utilised.

[lead chucker]

Thank you. That was good info.