308 Cartridge headspace

[mb121]

I recently got into reloading and am set up with a RCBS rock chucker, dies, powder measure, and scales (5-0-5 and chargemaster), frankford caliper. I have read the reloading 101 and 201, watched several videos on youtube, read the manuals/instructions for the dies/loading manual/etc., but have not found a answer for this.

Upon reading about bumping the neck back (.001-.002 for a bolt action) and learning about the datum point which for the .308 win is .400 and half way down the shoulder which is used to determine cartridge head space. SAAMI spec calls for 1.634-0.007, what I take away from this is according to SAAMI the spec is between 1.634 and 1.627. Being cautious, as I fully realize that reloading is an inherently dangerous if done incorrectly and I would like to keep my eye sight/fingers/life, on reading I purchased a Hornady cartridge head space gauge. Now measuring my once fired FGMM brass it averages out at 1.629, if bumped back .001-.002 then it is still within SAAMI. Where I stated to become confused was with I measured loaded factory FGMM 168 ammo that came in an average of 1.622 and Fed Fusion 165 gr that measured less than 1.620.

So my questions are as follows:

My recent FL sized brass ran between 1.623-1.622, with a few coming in at 1.620-1.621 which I set a side with intentions to discard. So with it being the same head space length as FGMM then it should be safe to shoot as long as I follow the proper reload steps correct? (load will be 2.005 trim, COAL 2.800, 168/175 SMK, IMR 4064 working from 41.5 to 45.0 gr)
What is the min/max length for head space?
Am I missing something about the head space?

If I am totally off base here please let me know as I am wanting to learn as much as I can, probably will have some more questions, but right now but this one has me pretty confused.

 

[roggom]

It is common to find factory ammo .004-.005 under Saami spec, this is so it will chamber in pretty much any rifle.

Doing your measurements you know where you need to bump, one or two firings should not be anything to worry about, but do what you must to keep the bump to .002 or it may develop a weak point near the case head.

Unless you are firing super hot ammo, I would just use them, but if in doubt throw them out.

 

[OF&B]

The Hornady head space measuring kit is a very flexible and useful tool for reloaders.

That being said, it is at it's best for relative measurements rather than absolute. Finding the sizing die setting to bump the shoulder back .002" it's fine for.

A PTG 1.630" 308 Go Gage measures 1.622" in my Hornady headspace measuring tool when the 0.400" insert is used. The 1.634" No Go Gage gives 1.626".

Small differences in the bore of the insert and any bevel on the edge change the measurement. I can get the same number every time, but that number isn't exactly the head space.

I've found some of my old sizing dies would bump shoulders back 0.020" too far, or .010" not enough when using the touch the shell holder method of setting the die, so this is a really useful tool to have.