Chamber Dia Question

[mdesign]

How much larger should your chamber be than the dia of your loaded rounds?

My loaded 7mm rounds measure .308 (.012 wall thickness) and the fired brass measures .315 across the neck. Not sure what the chamber measures but might guess that there could be .002 "snap" back after the round is fired making the chamber around .317 in dia.

This seems like a lot of clearance/expansion everytime the brass is fired.

 

[Victor N TN]

Re: Chamber Dia Question

Have you considered casting your chamber so you know exactly what it is? Brownell's sells an alloy just for that. If you are shooting a factory chamber I'll promise there is a lot of "slop" in it when using factory ammo.

 

[mdesign]

Re: Chamber Dia Question

I have never cast a chamber but it would be an interesting learning project. I should learn how to do this. Its a GAP chamber so I could also call them and see what their reamer measured.

SAMMI shows a .315 loaded dia for 7mm but you would have to have a .0155 wall to get that. I have not measured a case neck wall that thick in years, maybe never. Seem to me that factory brass is getting thinner and shorter all the time.

 

[918v]

Re: Chamber Dia Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mdesign</div><div class="ubbcode-body">Not sure what the chamber measures but might guess that there could be .002 "snap" back after the round is fired making the chamber around .317 in dia.
</div></div>

Its more like .0005" spring back.

 

[Winchester 69]

Re: Chamber Dia Question

Measure a fired case to get an idea of your chamber dimensions.

You want about 0.003" minimum neck-to-chamber clearance.

 

[mdesign]

Re: Chamber Dia Question

My chamber must be .316 dia which is the normal dimension for a 7mm match chamber from PTG. My loaded rounds measure .308 with a neck wall of .012 so it looks like a guy would want a reamer dia of around .312. That would be a tight neck. Wonder if that would work with Lapua brass?

 

[Winchester 69]

Re: Chamber Dia Question

Your chamber diameter should be a minimum of 0.335". Your desired bushing diameter should be 0.330".