Sharp throat edge

[cs86]

I've had trouble with a problem for awhile now on my 300 blackout and think I may have my problem figured out, but don't know how to fix it. I've been using powder coated boolits. The other day I wanted to do some function tests and found that my boolit was being scrapped by the edge before the throat (in the chamber). I believe this edge needs to be rounded or tapered in order to reduce the chance of the edge scraping the lead or powder coat before firing. Is there any way to smooth this edge out without affecting other parts of the chamber?

[williamwaco]

Are you talking about the case mouth or the chamber?

[cs86]

Sorry, I was having a hard time explaining it. I'm talking about the chamber. The edge that leads into the throat.

[runfiverun]

you need the throat reamed.
or to change boolits.
the rcbs 30-165 silhouette boolit will overcome your problem.

[cs86]

Here are some pictures before I cleaned the barrel.

156625
156626


Here is a pound cast I tried doing for the first time.

156627

The rough spot by the case mouth is the boolit grooves that didn't squeeze together very well.

[Gtek]

What about shutting the gas system and hand cycling a very careful few fire laps, might even clean up the pipe downwind.

[popper]

you need the throat reamed Pretty common complaint, maybe someone with experience could specify what reamer (PN) and how to carefully/successfully do the job?

[vzerone]

What it is, is the ejector puts pressure on the cartridge tending to tilt the nose of the bullet to the right at the 3:00 position. The nose is scraping on the sharp end of the chamber at the very end of the neck part right before the throat. You don't need to ream or polish the throat you need to break that sharp corner. I had the same problem with my AR. I was wondering why the rifle is grouping good and then one shot goes out the right every once in a while. It repeated that over and over until one day I extracted a loaded round and saw the nose had a long scrape at the 3:00 position. I polished that sharp edge and the rest was history, problem solved.

Another thing caused by that ejector putting pressure on the case are the two fine lines scarred on the neck after extraction. That is from the sharp inside corners of the locking lugs on the barrel extension. As the case is being extracted the angle of the case becomes more sharp and the neck gets dragged across those lugs right before it's ejected. All push feed rifles do this, but bolt actions, for an example, don't have any sharp edges in the chamber area, although they can have a sharp corner at the end of the chamber at neck area, but because they are manual actions the bullet nose doesn't seem to get snagged there.

Hope that helps. It drove me nuts till I figured it out and solved it.

[Char-Gar]

Sharp edges at the beginning of the throat and/or small burrs in the same place can be cure with a brass lap and 600 grit emery paste. This is a very mild abrasive that won't damage the chamber if cleaned well after the lapping.

About a hundred turns of the lap, cleaning and changing out the grit every 20 turns or so, will remove any roughness and put an ever so slight chamfer on the end of the throat.

A lap will have to be contrived, but cartridge cases are a good place to start. The shoulder of the case makes a good lap, but the lap needs to be held concentric in the chamber. Cases fired in the same chamber make a good bushing, if drilled out to receive the rod holding the lap.

[cs86]

You don't need to ream or polish the throat you need to break that sharp corner.

Yep, that's what I'm wanting to do.


can be cure with a brass lap and 600 grit emery paste.

I have the 600 lap paste. Just trying to understand how you are using a brass piece to lap. I'll try and play with a fired case to see if I can use it to take off the sharp edge.

If there are any other ideas on how to take this edge off please share.

Thanks for the help.

[geargnasher]

I have three of them and pound casts all look just like yours. I don't shoot PC bullets, though. If your bullets fit the throat and are sized to about a half-thousandth below throat entrance diameter, you shouldn't be having the scraping problem in the first place. You can use a lap (I have used a tapered steel punch more than once) and some VGC to cone the throat entrance a little, but unless your bullets fit the throat shape and size, lapping the entrance will likely only encourage misalignment. Bullet launch speeds are such that even a relieved throat entrance may not prevent scraping if you have misalignment of the drive bands with the throat entrance, or if the bands are too large.

Let me suggest a couple things. Using Federal 300BLK brass can solve some issues because it has thicker necks than anything I've seen made from cut-down .223 or 5.56. This helps the bullet stay centered, IF it's not larger than throat entrance to begin with. Seat the bullets out as far as you can without them sticking in the throat. Be careful using small-base AR dies, they can exacerbate the extractor-induced misalignment that Vzerone described. I use small RCBS base BLK dies, but the bullet pilots firmly in the throat, is sized .3095" for a .310" throat entrance, and I have modified the extractor slightly on my rifles so it hasn't been an issue for me. Lastly, you can just shoot the dang thing. H-110 and Lil' Gun are notoriously "hot" and a few hundred full-house supersonic loads will probably ease the throat entrance quite nicely.

Gear