Valve lapping compound to hone chamber

pidevildog07 · May 31, 2020

Is this a viable thing to do? I have a 308 ar barrel that either has a rough chamber, rough locking lugs, or both as evidenced by the unwillingness of new ammo to chamber without help and the accompanying marks that start at about 1/8" below the shoulder and end approximately the same distance from the case head.

I have headspaced (and it passes) the barrel and you have to help the go gauge into battery. I cringed when I realized it wouldn't come out with a simple tug and I basically had to tap it out. The same with the brass cased ammo. Something is amiss with the chamber. I already have a silicon carbide hone coming from Brownells to see if this remedies the problem.

outrider58 · May 31, 2020

I've used flitz and a dremel with a felt buffer head.

Mark75H · May 31, 2020

There are different grades of lapping compound. I would not use valve grinding lapping compound, Flitz sounds a lot better

John from MD · May 31, 2020

pidevildog07 said:
Is this a viable thing to do? I have a 308 ar barrel that either has a rough chamber, rough locking lugs, or both as evidenced by the unwillingness of new ammo to chamber without help and the accompanying marks that start at about 1/8" below the shoulder and end approximately the same distance from the case head.

I have headspaced (and it passes) the barrel and you have to help the go gauge into battery. I cringed when I realized it wouldn't come out with a simple tug and I basically had to tap it out. The same with the brass cased ammo. Something is amiss with the chamber. I already have a silicon carbide hone coming from Brownells to see if this remedies the problem.

Lapping can take metal off fast. You need to know what you are doing before using any kind of lapping paste. Valve grinding compound is way too course for lapping barrels and lugs.

Silicone Carbide is an embedding type of paste. You better get it all out or it will continue to cut.

Why don't you have someone borescope it first before doing anything drastic.

Qbeam · May 31, 2020

I've used it for a bolt carrier and upper issue, but not for a chamber. I would check it with a gunsmith or send it back to the manufacturer.

Q

Doco Overboard · May 31, 2020

I use everything from valve lapping compound to silicon carbide paste mixed with oil to smooth up a chamber.
Even down to Flitz or JB compound.
A chamber mop works good to get out lite scoring.
Sometimes a chamber reamer or a piece of leather rough side out will do the same thing.
Depends on what you find in there.
Flood it with olive oil if you make a lap out of a dowel and a piece of leather.
An Ar barrel should be relatively easy to manage because you can get right to it without dragging your paste all through the action.
Detail clean with solvent if you use carborundum or carbide grit.

pidevildog07 · May 31, 2020

John from MD said:
Lapping can take metal off fast. You need to know what you are doing before using any kind of lapping paste. Valve grinding compound is way too course for lapping barrels and lugs.

Silicone Carbide is an embedding type of paste. You better get it all out or it will continue to cut.

Why don't you have someone borescope it first before doing anything drastic.

Scoped it today and can see the rub marks and even a little brass shaving approximately 1/4" from the breech. Should be pretty simple. I've got honing oil coming too from Brush Research.

I already know what kind of damage can be done from over honing/polishing. I used to work at Benelli when we started disassembling M4's to send to Robar for np3 for the H20. The first batch that came back had the inside of the piston chambers np3'd as well as the whole gun save for the plastic parts and the sight assemblies. We assembled them noting that the pistons were difficult to install at first and test fired. Np3 confetti was the only way to describe what happened. The chrome lined barrel was np3! It was easy enough to clean the barrel out and then we discovered that it was also in the piston chambers. Cleared those with a flex hone to the point that the pistons wiggled way too much and could only fire Winchester Super Handicap loads reliably. Determined it was from gas blowby as evidence from irregular scuffing in the chambers from the piston lobes rattling around during firing.

Anyway, your suggestion is noted all the same. Nice to have a community such as this to check me from time to time. I've learned a lot more from screwing up though and would never work on anyone's stuff that I hadn't already screwed up on my personal firearms collection. In short don't let me work on your gun, lol.

John from MD · Jun 1, 2020

Well, it appears that you have the ability and knowledge. Go for it!

pidevildog07 · Jun 6, 2020

May have figured out my bolt locking into battery issue. The extractor has 2 orings that I'm guessing is not giving the extractor claw enough free movement to get over the rim of the case head therefore trying to just jam the case into the chamber and that's why it was rubbing in the lower left quadrant of the chamber. Is this normal for 308ar bolts? I've never seen them on Ar15 bolts until I added them myself.

Matlack · Jun 6, 2020

There is a special cutter or chamber hone is used for what you want to do. I would not recommend lapping the chamber. I dont have my notes from the SOTAR class I took. Ping Chad or wait for the right answer.

outrider58 · Jun 6, 2020

pidevildog07 said:
May have figured out my bolt locking into battery issue. The extractor has 2 orings that I'm guessing is not giving the extractor claw enough free movement to get over the rim of the case head therefore trying to just jam the case into the chamber and that's why it was rubbing in the lower left quadrant of the chamber. Is this normal for 308ar bolts? I've never seen them on Ar15 bolts until I added them myself.

It doesn't sound like honing will be a solution. AR chambers have a relief cut in the chamber to allow the extractor to open and close on a case rim once in battery. It's a little cutout in the barrel extension. Is that there?

pidevildog07 · Jun 6, 2020

Matlack said:
There is a special cutter or chamber hone is used for what you want to do. I would not recommend lapping the chamber. I dont have my notes from the SOTAR class I took. Ping Chad or wait for the right answer.

Too late. All the flex hone did was smooth out the chamber. The original problem still persists. I am starting to think that the mixture of components that I have all fit but will not function correctly. I just did see that the forward assist on my upper (dpms style supposedly) is near the ejection port not stock. The carrier has cuts on the side for a normal forward assist ( nearer to the stock).

I've seen two different pictures of "dpms style" carriers, one with scalloped side and one that's smooth. The one with scallops claims to be dpms style while the smooth carrier claims to be Lr308 style. Lr308 IS dpms correct? Are there dimensional differences I'm missing in the bcg that I have from Delton that would preclude me from being able to cycle the bolt into battery without hanging up? Now it looks like I have the wrong style of bcg. This shit is making me rip my hair out and now I'm not even sure I want to TRY to test fire this thing because I don't know what will happen.

pidevildog07 · Jun 6, 2020

outrider58 said:
It doesn't sound like honing will be a solution. AR chambers have a relief cut in the chamber to allow the extractor to open and close on a case rim once in battery. It's a little cutout in the barrel extension. Is that there?

Not exactly sure what you're asking me to look for so I took a picture. The second is of the supposedly "Lr308 style" bcg that came from Delton. The third is of the upper receiver. I thought Lr308 and dpms style were one in the same ad dpms came up with the name lr 308 in the first place. It feels like a "gotcha" right now. Not sure why all the images are flipped. Sorry

pidevildog07 · Jun 6, 2020

outrider58 said:
It doesn't sound like honing will be a solution. AR chambers have a relief cut in the chamber to allow the extractor to open and close on a case rim once in battery. It's a little cutout in the barrel extension. Is that there?

First picture is of Ar15 chamber. Second of the 308.

It may be because the Ar15 chamber is chrome lined (criterion barrel) and the 308 qpq (ballistic advantage) but from where I'm sitting, the Ar15 has a noticeably steeper angle in the breech than the 308. Is that what you're referring to?

pidevildog07 · Jun 6, 2020

outrider58 said:
It doesn't sound like honing will be a solution. AR chambers have a relief cut in the chamber to allow the extractor to open and close on a case rim once in battery. It's a little cutout in the barrel extension. Is that there?

Ballistic advantage does list their 20" 308 modern series as having a dpms style barrel extension.

outrider58 · Jun 6, 2020

pidevildog07 said:
First picture is of Ar15 chamber. Second of the 308.

It may be because the Ar15 chamber is chrome lined (criterion barrel) and the 308 qpq (ballistic advantage) but from where I'm sitting, the Ar15 has a noticeably steeper angle in the breech than the 308. Is that what you're referring to?

The cutout will be on the starboard side(not pictured).

pidevildog07 · Jun 6, 2020

outrider58 said:
The cutout will be on the starboard side(not pictured).

May look that way because my camera decides to flip the image. If it is not you'll have to show me an example if you could be so kind.

outrider58 · Jun 6, 2020

pidevildog07 said:
May look that way because my camera decides to flip the image. If it is not you'll have to show me an example if you could be so kind.

Sorry. It's the side where the extractor is. You'd almost have to take the barrel off the upper to photograph it. To be honest, I'm not sure .308s have it. Not all 5.56s do.

Matlack · Jun 6, 2020

What were you going to use to lap the chamber? A cartridge?

pidevildog07 · Jun 6, 2020

outrider58 said:
Sorry. It's the side where the extractor is. You'd almost have to take the barrel off the upper to photograph it. To be honest, I'm not sure .308s have it. Not all 5.56s do.

If it did I didn't notice it while I was honing the chamber. I'm assuming it would be very obvious correct? I ask because I've seen extractor cuts on shotguns and bolt actions but of course not the same on this platform. I did use an endoscope to look at the part that's obscured by the upper like you said and it didn't look any different than the rest of it, but if you look back at the pictures there's a very obvious rub on the breech face but it doesn't coincide with where the bolt was rotating inside the chamber but the rub didn't have any depth to it. I almost feel like I have a dpms carrier and an armalite bolt :confused:

To be honest if it will chamber rounds and passes headspace it may just be a "wear together" issue that's less than common for bolts and barrels purchased separately and/or barrels headspaced at the time of manufacture. I'll just shoot the piss out of it and hope it doesn't tear open a portal to the Astral plain :gun7:

Matlack said:
What were you going to use to lap the chamber? A cartridge?

60 grit sandpaper lol. Nah I used a 308 caliber flex hone and ass loads of oil.

Valve lapping compound to hone chamber

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