20BR vs 20BR-IMP for 55gr Berger

[gunderwood]

I'm thinking of building a benchrest 20BR and I would like to run the Berger's in a 1:9 twist. From what I have read, 26-27" is plenty of barrel for a 20BR (i.e. going over that doesn't give much more velocity). I would be setting up a dedicated varmint rig.

My questions are:

1. The 50gr Berger seems to be a good match for 20BR, but how is the 55gr stacking up? Is the extra case capacity able to compensate for the extra 5grs of bullet weight? I've heard the 50s could be pushed to around 3800fps.

2. I generally haven't messed with fire-forming brass and such with varmint guns because for an extra 100-150fps it doesn't seem like the hassle is worth it. However, given that I'd buy 6mmBR brass and neck it to 20 cal, I assume that I can leave a step in the neck for the first firing. If so, that wouldn't be much work.

Ideal powders? Thanks.

Edit: What is the die situation?

 

[cmillard]

might as well try the improved version. no drawbacks to doing it. i think that powders in the range of 4895, 8208, and h335 would be ideal for this round. i think you will be on the edge of hitting 4000 fps with the 50-55 grain bullets in the improved version. that will shoot like a laser out to 400 yards.

 

[gunderwood]

There are negatives to pushing a bullet too fast. I've heard of some reports concerning the 20x47 pushing a 55gr Berget past 4000fps not killing coyote's quickly. The bullet never penetrates because it blows up on the surface.

Other potential negatives:

1. Increased throat erosion as it is more overbore.

2. Decreased barrel life due to bullet velocity.

3. Increased cost of reloading (should be trivial unless dies have to be custom made, which is likely).

4. Increased work to get brass (fire forming).

All of that may be worth it, but that is a lot of potential negatives to find out your pushing the bullet too fast so it isn't killing things quickly.