Issues with new Monster build

[nexgen91]

Wondering if someone can help me out. I got a chance to take my Monster Grendel build (20 inch barrel with matching bolt) out to the range over the holiday, and I'm running into some issues. The bolt is having a hard time getting fully into battery most of the time (won't go forward with forward assist either), I have tried both factory ammo and reloads (checked with a lyman case gauge). I have successfully got it into When battery a few times but when I fire a shot I am getting a lot of light primer strikes, and then failures to come out of battery resulting in me having to use a mallet on the charging handle to get it to eject (this happens when cycling rounds without firing also). I have tried two different bolt carrier groups, two different lowers (a rifle and a carbine) and am running out of things to try. I also tried multiple magazines also. I cleaned out the chamber good before range, and gave it another it another once over prior to troubleshooting. It seems to be cambering ok with no cartridge loaded, the bolt face depth is measuring around .126 to .134 depending on which lug I measure off of. The lugs themselves range from .279 to .286 inches. When placing factory loaded cartridges in the chamber and pressing in, and they will not fall out under their own weight, I tried tapping on the table, and most of them still will not come out (a few did drop out with considerable force). I had to push from the other end to get the others to come out. I have tried federal and hornady ammo both of them produce the issue, do not have any wolf on hand.When trying to cycle a magazine one or two will eject with the bolt, but then it will get hung up. The rounds seem to be chambering sometimes, and then sometimes they won't. I was able to fire it a few times in a row yesterday, and then it would get stuck. I don't see any wear that would indicate the gas key hitting the gas tube. I'm thinking there might be some kind of headspace issue, but I'm not sure. Any suggestions?

[FLshooter]

The lower is definitely not the problem.Sounds like the bolt is fine too.From what you described,it sounds like a tight chamber.Especially if you are struggling with the charging handle just to get a round to eject.

[Lemonaid]

You can learn a lot from the fired brass, take measurements and compare to the dimensional drawings. Also check if there is "Frosting" or chamber reamer marks that show up impressed in the brass = Rough chamber.

Take a black marking pen and coat a factory unfired round, bullet and case, then chamber and eject without firing (Do this at the range pointed in a safe direction). That can show if bullet is hitting lands (short chamber or bad factory ammo loaded long, bolt barrel mismatch ) or if it is the shoulder hitting, or the neck is tighter than it should be. That should get you more info about what's going on.

[grayfox]

Rounds, especially factory, that won't chamber/give light strikes has happened to me in the past, in one barrel, not a monster btw... it was a short chamber/too long of a shoulder on the brass. But yours sounds more like a non-std bolt issue.

The bolt face depth is either 0.125 or 0.136, +/- a 0.001, you can't have both. Carefully re-check, don't measure off of the extractor. If you have an in-spec chamber sized for 0.136 bolt face depth (the monster barrels are typically sized for 0.136" bolt face depth bolts), but your bolt face is 0.125 (ie, a bit more shallow than the real grendel 0.136), that's the root problem. A 0.136 chamber will be 0.011 shorter than a chamber that uses the 0.125 (which your bolt might be). So the brass won't have enough room to seat. Measure your bolt face depth to see if it is 0.125" or 0.136".

Hope that all makes sense.

[nexgen91]

Thank you all for the tips. I went back and verified the bolt face depth it is .136 +/- .001 I think the bolt is ok. I will have to cycle a black colored round in the morning and see what I get, will report back once I do that.

[nexgen91]

I cycled a round after coloring it black, and it is showing some wear on the brass. I also ran my borescope in the chamber and see this: https://imgur.com/a/GoN2oHK looks a little rough to me.

[bbqncigars]

This could all be due to a worn chamber reamer. That would give rough undersized chambers. I'd see about sending the barrel back myself, unless you're into doing some polishing.

[nexgen91]

Got the barrel and bolt headed back for an exchange, thank you all for your help.