Gunsmithing Good dimension to set throat at in new barrel??

[Widowmaker]

I'm having a smith blueprint the action on my SA Remington 700 and rebarrel it with a new, 22", 11.25 twist, Bartlein barrel. The barrel will be chambered in 308 Winchester. He's asked that I send him a dummy round that he will then use to tailor the chamber to my reloads. They will consist of a 175 gr SMK in a Lapua case using Varget and 210M primers. My question is what should I set the COAL at to where it touches the lands? Where would you set it if it was your rifle? I've already checked my rifle and the maximum COAL that will fit in my magazine (AICS) and reliably feed without hitting the feedramp is 2.875". I would like to keep it short enough that I'm able to chase the lands as the throat erodes. I was thinking of setting the COAL at 2.820". If the bullet is touching the lands at that point, I can back off to 2.800" when I start doing a OCW workup on the rifle. Then when I have a good powder charge identified, I can do a seating depth test from touching the lands back to .040" or so off the lands. Setting the COAL at 2.820" (touching the lands) would leave me .055" to be able to chase the lands as the throat erodes and still be able to feed from the magazine. Does that sound feasible? Your thoughts on the matter would be appreciated! Thanks

 

[Threadcutter308]

I'm having a smith blueprint the action on my SA Remington 700 and rebarrel it with a new, 22", 11.25 twist, Bartlein barrel. The barrel will be chambered in 308 Winchester. He's asked that I send him a dummy round that he will then use to tailor the chamber to my reloads. They will consist of a 175 gr SMK in a Lapua case using Varget and 210M primers. My question is what should I set the COAL at to where it touches the lands? Where would you set it if it was your rifle? I've already checked my rifle and the maximum COAL that will fit in my magazine (AICS) and reliably feed without hitting the feedramp is 2.875". I would like to keep it short enough that I'm able to chase the lands as the throat erodes. I was thinking of setting the COAL at 2.820". If the bullet is touching the lands at that point, I can back off to 2.800" when I start doing a OCW workup on the rifle. Then when I have a good powder charge identified, I can do a seating depth test from touching the lands back to .040" or so off the lands. Setting the COAL at 2.820" (touching the lands) would leave me .055" to be able to chase the lands as the throat erodes and still be able to feed from the magazine. Does that sound feasible? Your thoughts on the matter would be appreciated! Thanks

You've done your homework! I've been shooting 168 gr Matchkings @ 2.800" and your math makes a whole bunch of sense. @ 2.800" or 2.820", you should have plenty of room for land chasing.

 

[Jibnast]

Why not have him notch the feed ramp. I have not played with the 175 SMK in a while but I'm thinking you might be having the base of the bullet in the shoulder area.

 

[Widowmaker]

Why not have him notch the feed ramp. I have not played with the 175 SMK in a while but I'm thinking you might be having the base of the bullet in the shoulder area.

I had thought about possibly having him notch the feed ramp but I'm not sure it's worth it. With a 308 Lapua case trimmed to 2.005", the heel of a 175gr SMK bullet seated to a COAL of 2.810" is just ever so slightly above the neck-shoulder junction of the case. With the bullet seated out to a COAL of 2.875", there is approximately .200" of bullet bearing surface left in the case neck and the base of the bullet is right at the neck-shoulder junction. At that length, the round still feeds fine from my magazine. I'm not sure I'd want to seat the bullet out any further and have less of it inside the case neck. The AICS magazines I'm using have an internal length of 2.880" ( only .005" larger than maximum COAL of 2.875" that will reliably feed through my rifle now). I do have two of the Alpha magazines and they have a larger internal length of 2.965" which would allow the bullet to be seated further out another .085" or so. But in doing this, I don't think there would be enough bullet left in case neck. On the other end of the spectrum, what do you think the shortest COAL is that the 175gr SMK should be seated? At a COAL of 2.810", the heel of the bullet is just slightly above the neck-shoulder junction with the boat tail inside the case. How short can the bullet be seated without causing any pressure issues? COAL of 2.790"-2.760"??? I would like to be able to keep the velocity on the rounds up around 2500-2700fps since I do shoot somewhat regularly out to 1000yds. Will seating the bullet deeper affect the maximum velocity I'll be able to obtain since there is less internal volume for powder inside the case?