.300 wm fgmm

[300Win492]

Hello all! Thanks in advance to any experienced reloaders who choose to reply. I am fairly new to reloading and have set out to replicate FGMM 300wm 190's. I have had good results with this load out to a 1000 yds. I have recently picked up the RCBS Precision Mic. to not only replicate the load but develop the correct load for my chamber. I put my last 15 FGMM factory loads in the headspace gauge and they all came back .015 below sammi minimum according to the gauge. All the once fire brass comes back at .002-.003 over sammi minimum according to the Rcbs mic. At this point I'm wondering two things.....

One - to anyone that has used RCBS Precision Mics... How accurate are they?

Two- on the assumption that it is an accurate measurement...Do I set up the sizing die to sammi minimum and bump the shoulder back .002 or do I push the shoulder back closer the rounds factory dimensions BELOW sammi minimum?

 

[rcmodel]

Neither.

Just push them back to where the bolt will close on then with very slight drag.

That will be 'perfect' for your guns chamber.

rc

 

[Bart B.]

Set fired case shoulders back .002". Then use this belted case collet die (www.larrywillis.com) to size the case body right in front of the belt. This produces best accuracy with belted cases. Having the bolt face squared up helps a lot, too.

New belted cases typically produce better accuracy if the bolt face isn't square with the chamber axis. They also can equal the accuracy of correctly resized fired cases.

If your bolt binds even a little chambering a round, accuracy will suffer. The bolt has to go back in battery exactly the same for each shot. It don't if it binds. This gets worse if the bolt face isn't squared up.

No bottleneck case is 'perfect' for any chamber unless its first fired in a chamber after the bolt face is square and is always minimally and properly full length sized to have .001" to .002" head clearance every time so the bolt never binds and goes back into battery exactly the same for every shot without interference. If the bolt face ain't square for every shot fired for each case, the case head and bolt face will have different interference fits every time fired and the result is different barrel whip amounts and directions relative to the line of sight as the bullets leaves. Accuracy suffers.

Your new FGM cases' headspace may not be at SAAMI minimum (2.263") nor maximum (2.270"), but somewhere in between. I think the Mic is accurate +/- .001" from SAAMI minimum chamber spec 2.279" when it's at zero. Use it to measure your fired case shoulder setback. Your chamber's breech to shoulder dimension is about .001" more than that of a fired case. Chamber SAAMI maximum is 2.289".

 

[300Win492]

RC & Bart thank you for your replies