300 BLK Headspace Problem

[EricBu]

My brass was converted by Top Brass from Lake City 5.56 NATO cases.
So it appears I now have three alternatives to try:
1. Try a different brand of magazine
2. Tighten my small base resizing die just a tad to get the head very slightly below the level of the case gauge (unfortunately the RCBS gauge doesn't have the two levels that a Wilson has.. only flush or not flush).
3. Loosen the seating die. Also check the powder feeding die which, I believe, does a slight case neck expansion.

So......if you have Top Brass converted........that's sized out of the box (can't convert without resizing it). So you are sizing it again, which is fine. But you could just back your sizing die all the way out and just load the Top Brass out of the box. But first.....do a little experiment. Take one that you are sizing, without putting a bullet in it, and plunk it. Then compare it to one of the Top Brass pieces out of the box before you resize it, plunk it. See if there is a difference in how they drop. I've never used any of Top Brass's converted brass, but they have a good reputation. If it's all LC, thickness isn't an issue either. Oh.....and get a Wilson, Lyman, or Dillon case guage. Not having a min and max is like using a dial caliper with one arm.

 

[MFInc]

I'll be doing my first 300 aac rounds here soon. Lots of good info here to soak up. I have some questions on this round, but will try find the answers before I create a post.

 

[cfullgraf]

Side note.

Most case gauges are cut generously in the body dimension.

A Sheridan gauge is the only I know of that is cut to SAAMI chamber specs.

So a cartridge could fit the gauge and not chamber.

 

[KentM]

- Does a resized (but empty) case fail to lock up ? (just hand feed the empty case/let the bolt slam home)
- Or does it freely go into full battery when the bolt closes? Can you easily eject the case manually?
(sizing's fine, something happening upon bullet seating)

Good idea. I will check that when I get the gun back from my son, who has it right now.

 

[rbernie]

Measure the neck thickness for 10-12 brass after resizing and see if they’re within SAAMI spec.

 

[mcb]

Stupid question but did you gauge the brass AFTER seating bullet? You can very slightly squish the shoulder and cause condition you're describing - especially if it's intermittent. No inside mouth chamfer or rough trim operation + flat base bullet, can put a tiny bulge where the case starts to taper into the neck and it's very easy to not see it. This is an exaggerated example (middle round), but you can do that very slightly & get random rounds that won't chamber & easy to not notice when handling the finished rounds.

Edit to add: what specifically does JAM mean in your case? What I pictured will feed just fine but bolt won't close and if you have a strong enough buffer spring or try forcing into battery with forward assist, it will stick in chamber & need rodded out.

If JAM means they fail to feed, a 110 VMax under mag length should get gobbled up. So if that's the case - depending on what generation PMag you're using and if you shaved the rib a little if an early generation - try a Lancer L5 AWM. Those are excellent feeding 300BLK. From there, check barrel extension clocking, feed ramp burrs, etc

View attachment 1007392

Little late replay but I had this exact same problem loading the 125gr V-max bullet. I backed off my crimp a touch and everything works great now. I am loading the 125 gr bullet to an OAL of 2.060 in.

Others have alluded to this but there are two different common gauges. Case-Gauges are there to check that you trimmed the case to the right length. Then there are Chamber-Checker gauges that are there to check loaded ammo. Case Gauge diameter does not have to be at SAAMI max material (smallest ID) but a good chamber-checker gauge will be machined at max material conditions. By being as tight as allowed the chamber checker ensure that if it goes into and easily out of you chamber-checker then you know it will work in any firearm properly chambered for that cartridge.

 

[jmorris]

If factory ammo is fine it’s not the magazines. Case gauges are not all the same and don’t matter at all if the round won’t fit the rifle.

Try this.

 

[MEHavey]

I strongly recommend Full Length re-sizing for gas guns.
No "fiddling" with "just enuf" shoulder set-back.

It just ain't worth
- (Best Case) a wasted trip to range when rounds won't change
- (Irritating Case) where stuck rounds FTF and have to be mortared out (Thank you Gene Stoner for a fail-safe design)
- (Worst Case) come unglued OAB (Garand-type designs)

Fire it a half dozen times, checking for stretch/paper-clipped, and toss.....
Especially when dealing with Garand variants