Lesson Learned and Some Improvisation

[JohnCarter17]

I present:

Exhibit A (BCM Blem)

Exhibit B (ARP CLE 12.5")

Exhibits FC and WW,CD (repurposed stick o wood/dowel and unloved Olympic Arms barrel)


So I had the ARP barrel in the freezer for a good 4 days.
I assembled the upper (door removed since) tried to true the receiver face but the Brownells receiver lapping tool wouldn't slide in. Maybe I wasn't using the proper finesse.

Anyway I said fuck it, lets assemble it. That was at about 3:00 AM after being woken up to perform a chore.


After using a heat gun, I failed to get my barrel in (insert That's What She Said joke here). Heated again, finally got it in, but didn't get the pin aligned.

Fuck me.

So I tried hammering with a rubber mallet using the dowel (Exhibit Fail City), while the upper was still a bit toasty, but got nowhere. The dowel had been recommended and successful in non-thermal fitting barrel removal duty, to break the Loctite seal, etc. Trash can under barrel to catch it.

I tried again today, with feeling. Still no luck. I had been contemplating what bar stock or metal I had. Then I remembered the "shot out" Olympic Barrel (Exhibit Winner Winner, Chicken Dinner) that a buddy gave me after he replaced it with a Larue. At first I am like I might need to remove the MD, but no, stupid, thats narrower and you expose the threads to the upper. I double checked and it was seating nicely.

Sure enough after 5 blows it was starting to move and I got it out.

Hot damn. I was able to put that Olympic barrel to a good purpose, and now I have another tool in my arsenal and remove a thermal fitted barrel much quicker in the future.

The ARP barrel is back in the freezer.

I really need to look again and see why the receiver facing tool didn't seat in the upper.
And clean up some splinter debris out of the upper and barrel.

 

[346ci]

<<Another ARP fan, good stuff.

I use temperature and a shop press, makes it quick and easy.

 

[xciapup]

When you say you are heating the upper, to what temp?

 

[JohnCarter17]

I use a heat gun, for probably 30-45 seconds, max setting to the threaded end. This cause enough expansion so the barrel slips in easily.

 

[JS8588]

If the thermal fit is that tight, get it started & lined up so the pin is aligned with the notch for it. Then, get your barrel nut started on the threads. Screwing on the barrel nut will push the extension the rest of the way in. Used this technique several times with success.

 

[Trigger Monkey]

Or use a torch

 

[Halfnutz]

Freezer is not cold enough imo. In the shop we have had industrial freezers that go well bellow residential types. Ours have not lasted long though, so we have a large container of dry ice we soak items in. Steel has an expansion coefficient of around 10.-11.5 per.degree Celsius. Aluminum is 25.5ish. The colder you get the barrel extension, the better off you'll be.

 

[bfoosh006]

When you say Browning tool ... do you mean the Brownells lapping tool ?

 

[xciapup]

I use a heat gun, for probably 30-45 seconds, max setting to the threaded end. This cause enough expansion so the barrel slips in easily.

When I do this, I have a lazer pointed IR thermometer in one hand, and a map gas torch in the other. In this way, I know what my temp is. If you don't have a thermometer, then you don't whether you need to apply more heat, or if you are not there yet. The most dangerous is to have overheated aluminum. That is dangerous. Too low a temp can just sabotage your project.

As to the lapping process. There are various OD dimensions for lapping tools. Either buy the correct one, or see if someone will loan you one, or have a machine shop modify yours.

If you lap, then you also need the skills and the tools to shape your barrel extension ramps to your M4 ramps on your lower. Remember, your extension ramps will overlap the M4 ramps because you shortened the distance by the amount of material you had to take off of the upper.

If you lap, then you also need to use a bedding compound to separate the fresh upper bare aluminum from the steel extension.

Lapping takes more skill and more tools than most people realize.

 

[CaptNemo]

I had to reduce the OD of the Brownell's lapping tool about .002" to get it to slide into a BCM upper receiver.
It works on all of the AR15 receivers now.
It you are removing enough material that the M4 Cuts need to be recut, then you either are buying really poor quality uppers, or you're really going after it too hard.
Normally about the 3rd or 4th touchdown and check and I have the receiver faced.
Unless you're submerging the rifle in sea water or some really nasty environmental conditions, the Aeroshell grease you use on the receiver treads will seal off the light ring of bare aluminum from corrosion.

 

[JohnCarter17]

This really is a weird exception case. I have done this 4 times before without issues: 3 Lilja, 1 Roscoe into 1 Noveske and 3 BCM.

The same with using the lapping tool, I think I need to make sure it's clean.

OTOH I can also try some spare BCM uppers to see if it's just this one. I have this problem, I keep buying a couple when they are on sale.

I can also use some calipers on the ARP barrel extension and see what it measures.

I have an IR thermometer gun I can use, I didn't think of utilizing it outside of brewing and cooking.

 

[JohnCarter17]

When you say Browning tool ... do you mean the Brownells lapping tool ?

Ack, yeah the Brownells tool, my bad.
Edited for posterity.

I also have 3 different grit compounds. In general I am very timid lapping.

I will align the upper vertical and level. Apply a thin amount of compound, insert tool and hand rotate maybe 10 times clockwise then 10 times counter clockwise. Cleanup then proceed.

 

[JohnCarter17]

Data point.

This latest batch of BCM blem uppers (full keyhole symbol and marked BCM4 under gas tube inlet) are too narrow to fit the Brownells receiver lapping tool. I cleaned the tool with isopropyl and tried the upper in the OP, and another unused from the same order. By full keyhole, the bottommost horizontal line of the keyhole is there, closing the symbol outline. I have two of these from a December order.

Some older BCM uppers, which also say BCM4, but the key hole isn't closed at the bottom, will fit the lapping tool.

Interesting. So I can't lap these 2, but damn they have a tight lock up.

 

[6.5SH]

A tip for lining up the index pin when you need to thermo-fit. Take a sharpie and a straight edge and make a line on the barrel extension.
Makes it super easy to keep things lined up when working fast.