Pin and Weld drill depth - potential problem?

[Antonio5005]

Did a pin and weld on a BCM 14.5" M4 barrel of mine the other day,. Accidentally went .100 deep with the drill. Depth measured from drill tip to top of threads. A little math shows I have roughly .040 between the barrel ID and drill point. Unfortunately, I didn't setup my dial indicator correctly, and when double checking to make sure I went deep enough, I noticed I went deeper than I had intended. Excuses, excuses... I was aiming for about .070 deep. Does anyone see this as a future problem? Could any accuracy issues arise from this? Drilled the hole with a #41 drill bit (Ø.096), 118° point. After triple checking inside the barrel best I could, I did not see any dimple where the drill point may have pushed the material inward. So it seems safe to assume there wont be any issues but wouldn't mind getting an extra opinion or two. Thanks!

[GH41]

If you haven't welded it yet do this.. Put a 118 degree point on the pin and turn it for a snug fit. Tap (don't beat the shit out of it) it in until it bottoms out and weld it. This will offer some support to the thin spot.

[Antonio5005]

If you haven't welded it yet do this.. Put a 118 degree point on the pin and turn it for a snug fit. Tap (don't beat the shit out of it) it in until it bottoms out and weld it. This will offer some support to the thin spot.

That is a great idea! But... yea I’ve already welded it over. I was in bit of a rush that day and figured it was already drilled, and luckily not drilled through, so why not finish it. Not like I could hit an undo button. Hasn’t been shot yet though.

I could.... machine it back off and then do your idea. Then put a new flash hider on, drill and weld it over again. Basically plug it and redrill.

Seriously wish I would have tried your idea first.

[lysander]

I wouldn't even worry about it.

The stress even with that thin a wall will be around 50,000 psi. The yield strength of annealed barrel steel is round 55,000 psi. After heat treating that figure goes to around 90,000 to 100,000 psi.

ETA: If all a barrel had to do was contain the pressure, they could be very thin, the only reason barrel are as thick as they are is so they are stiff.

[GH41]

I wouldn't even worry about it.

The stress even with that thin a wall will be around 50,000 psi. The yield strength of annealed barrel steel is round 55,000 psi. After heat treating that figure goes to around 90,000 to 100,000 psi.

ETA: If all a barrel had to do was contain the pressure, they could be very thin, the only reason barrel are as thick as hey are is so they are stiff.

I am not qualified to dispute your answer but I do want to understand what you said. If what you say is true can I assume I can build a spray and pray barrel with a wall thickness of 40K??

[lysander]

I am not qualified to dispute your answer but I do want to understand what you said. If what you say is true can I assume I can build a spray and pray barrel with a wall thickness of 40K??

"Spray and Pray" as in full-auto probably not, as soon as it heated up, it would begin to droop and probably get a bullet out the side. Like I said "If all a barrel had to do was contain the pressure"... a barrel has to to a few more things than just contain pressure, for one thing it has to absorb heat, and a lot of it. However in the case stated in the OP, we were talking about a .125" diameter section, less than 1/2 inch from the muzzle, and surrounded by not only the rest of the barrel material, the muzzle device as well.

Just to contain the pressure a barrel with a wall thickness of 1/4 inch around the chamber and tapering down to a wall thickness about 0.020 at the muzzle, would do it but that is a safety factor of 1.

Remember, that 60,000 psi chamber pressure is at the chamber, by the time the bullet gets all the way to the muzzle the pressure has dropped to less than 15,000 psi.

Armalite back in the 1950 made a sniper rifle that had an aluminum barrel with a steel liner, the liner had a wall thickness of about 1/16 inch. These worked reasonably well, except when the idea was put on the AR-10, it didn't handle full-auto too well.

[Antonio5005]

I wouldn't even worry about it.

The stress even with that thin a wall will be around 50,000 psi. The yield strength of annealed barrel steel is round 55,000 psi. After heat treating that figure goes to around 90,000 to 100,000 psi.

ETA: If all a barrel had to do was contain the pressure, they could be very thin, the only reason barrel are as thick as they are is so they are stiff.

That’s true, I didn’t really think of that. And like you stated as well, that is at the chamber, not the muzzle.

I did get test fire today, no issues of course, checked the barrel to see if there was any deformation inside of it at the pinned area and there was none. Used a geissele ssa trigger and a 3-15x scope to test fire at 100yd. Only had 55gr pmc xtac with me, it made a group around 2 or 3 moa or so. I just eyeballed the grouping size, no official measurement as that grouping seemed about on par for what I’d expect to see. I’d like to get some mk262 or something match grade and check but probably not necessary.

I do feel much better about it now since getting some extra opinions on it. I’ll try it with some better ammo for fun but it would seem i’m in the clear.

Now... how will a suppressor affect it lol. I’m joking...

Sorta... I don’t have a suppressor though, want a surefire for the 3 prong I installed on it.

[1911-A1]

Does the NFA have regs on blind pinning aside from using 1100 deg solder? Pin diameter? Depth of hole?

[GrumpyM4]

I wouldn't even worry about it.

The stress even with that thin a wall will be around 50,000 psi. The yield strength of annealed barrel steel is round 55,000 psi. After heat treating that figure goes to around 90,000 to 100,000 psi.

ETA: If all a barrel had to do was contain the pressure, they could be very thin, the only reason barrel are as thick as they are is so they are stiff.

Yes and no.

The MIL-C-63989C says that M855 shall not be less than 12,000psi average at the port on an M16A2.

The bore pressure near the muzzle is nowhere near the same as the chamber pressure, so the pressure the pin and weld job would be far, far less then 50k PSI. See the chart below for bore pressures at the port for various length gas systems.

Gas port pressure by system length.jpg

That being said, thin walls will expand and contract more as the bullet passes and attempts to push the barrel material out of the way so deformation, cracking, flaking and eventually material removal will happen relatively quickly at that spot. This could end up marring the bullet as it passes that point and will also lead to excessive erosion at the muzzle, both causing accuracy issues. Best not to run this way with a can.

I've had a smith accidentally drill all the way through into the bore on an MR556 pin an weld job nearly a decade ago. I had them counter bore the muzzle to 6mm ID and 5mm in to remove all contact and stress from the bullet, basically making a 90 degree crown set back by 5mm into the end of the muzzle.

Ended up being a tack driver and could print sub 1/2 MOA groups with the right ammo, trigger, and optics. Suppressed just fine as well.

[Antonio5005]

Yes and no.

The MIL-C-63989C says that M855 shall not be less than 12,000psi average at the port on an M16A2.

The bore pressure near the muzzle is nowhere near the same as the chamber pressure, so the pressure the pin and weld job would be far, far less then 50k PSI. See the chart below for bore pressures at the port for various length gas systems.

Gas port pressure by system length.jpg

That being said, thin walls will expand and contract more as the bullet passes and attempts to push the barrel material out of the way so deformation, cracking, flaking and eventually material removal will happen relatively quickly at that spot. This could end up marring the bullet as it passes that point and will also lead to excessive erosion at the muzzle, both causing accuracy issues. Best not to run this way with a can.

I've had a smith accidentally drill all the way through into the bore on an MR556 pin an weld job nearly a decade ago. I had them counter bore the muzzle to 6mm ID and 5mm in to remove all contact and stress from the bullet, basically making a 90 degree crown set back by 5mm into the end of the muzzle.

Ended up being a tack driver and could print sub 1/2 MOA groups with the right ammo, trigger, and optics. Suppressed just fine as well.

Counterboring it back sounds like a good option and I can see how much better it would be, but for this particular barrel, I don't see myself doing that. I do, however; have access to a lathe and can cut the barrel to 11.5 or 10.3 and re-thread. Im currently in the middle of a pistol build as well and have not picked out a barrel for it yet... Good opportunity? I have cut a barrel down like that before with great success. Just not sure that it's worth it for this one. Will mine hold up for the long run, or will it slowly start giving me problems? That's kind of been the debate running through my head since I first test fired it. As usual, decisions, decisions...