300blk potential brass base swelling issue

[Brody760]

to start out ive got a 10.5 1/8 300blk ar pistol i have been chasing my tail with what i thought was a headspace issue after more testing and some help from some amazing people and discovered what i think is a base swelling issue it wont let me post pictures from my phone so i will just have to describe it i have a piece of brass at a headspace of 1.076 and a .200 line diameter of .372 this piece of brass DOES
chamber but is barely sticky then i have a second piece fired from the same rifle that measures 1.072 after being sized back as i thought it was a headspace issue and a .200 line of .375 this piece does NOT chamber and now why this matters is ive had some overpressure rounds come out of the same gun at a headspace of 1.081 so i think even the 1.076 isnt as high as the headspace can go but i cant shoot anywhere past that because the base is starting to get swollen now DISCLAIMER this was with some mixed brass that was fired in my same gun but some was overpressured

i am going to try some strictly factory brass to see if it swells after a few reloads (subsonic) i will update this thread once that has been done but im posting now because my hornady custom grade die cannot seem to get down far enough to size the base without bumping the shoulder back too far so if this is the issue would a small base die fix my problem? and if so ive read some info on small base dies and heard that they size the shoulder back to minimum spec i dont want this as i want a .003-.005 thousands bump from my chamber fire-form measurement so is this just a myth or if i have base swelling issues then id have to size back my shoulder even with a small base die?

also one last thing, i know its close but aren’t my .200 line numbers under max spec? maybe i just have a tight chamber?

UPDATE: after shooting white box winchester subs for the brass i have a piece of brass that measures 1.0765 for headspace .375 for rim diameter and .3705 for the .200 line
measurement. This case requires a little pressure to push the bcg in by hand and requires a fair amount of pressure on the charging handle to extract the round

I have a second cartridge still new white box brass and it measures 1.074 for headspace, .375 for rim diameter, and a .200 line of .3075. This piece chambers and extracts with little to no pressure at all. ive had a over pressure load shoot a piece of brass out at 1.081 with a swollen base and blown primer but thats with a pistol length gas system and a big ol suppressor so do you think the 1.081 was just a over gassed anomaly or something else

the reason im asking this is where brass starts to get sticky at 1.076-1.077 is still short for headspace in a 300blk rifle right? so does this mean i have a tight chamber or something else?

[20X11]

Generally speaking, yes small base (SB) dies were designed to handle this. Mixed headstamp may include 5.56 bra6s that was fired from a machine gun (notorious for having this base issue), then chopped down and sized for 300blk. Or you can buy a roll sizer...or buy brass from HOSER or have Hoser process your brass.

[Brody760]

Generally speaking, yes small base (SB) dies were designed to handle this. Mixed headstamp may include 5.56 bra6s that was fired from a machine gun (notorious for having this base issue), then chopped down and sized for 300blk. Or you can buy a roll sizer...or buy brass from HOSER or have Hoser process your brass.

Please check the update to the thread.

[dellet]

There are more places where the diameter can be off than just the 200 line. The neck, Base and shoulder are common area’s.

You need to check also for out of round.

A small base 223 die will not touch the shoulder.

[20X11]

So, I also need to ask what your trim length is after sizing?

[Brody760]

So, I also need to ask what your trim length is after sizing?

1.358 to the hornady manual