SRC M4 Upper/Lower Joy

[sixb0nes]

Hey folks,

Finally got around to opening this recent trade (SRC M4 SD GEN III) up for some cleaning, and to put in a 363mm inner barrel. I noticed some upper/lower separation early on but didn't put any thought into it.

Now that I've opened it up, I'm seeing the problem. With these SRC bodies (I don't know how common this is as I'm not a big M4 collector yet) it appears there are no lower tabs to allow for a pin through on the rear of the receiver. The upper appears to stay in place both by the conformed shape around the gearbox and a ring of tongue/groove around the rear which lines up to the upper receiver.

See pics below ...

Notice the rearmost bolt/pin are still in place - no need to remove as there are no tabs. Also notice the broken tongue that was obviously sheered off by the previous owner unbeknownst to me. This tongue should pretty much encircle the upper part of the buffer area, but in my case there is only a few mm left of coverage:





This next picture shows where the groove is on the upper that would fit up to the tongue on the lower. There are also two set screws there which I haven't muddled with, but you can see a square tab on the upper that may coincide with them, again to prevent movement (I think):



Back together and see the gap:



Now after squeezing:



So. Kinda sucks. I had a few ideas in mind to alleviate it the movement aspect, as seen between the last two pictures; but, I wanted to hear from the community or maybe someone who runs this configuration and has experienced this and how they solved it. I don't know what kind of force was exerted here to break it ... so this may not be common at all? I also don't think this is a common design, but again, I could be wrong.

My thought was to run something like thin 3M tape along the inside of the upper, away from any moving parts. I think that would cause a permanent gap (or not depending if I mount tape on upper or lower part of curve), but better than movement.



Hope someone has some ideas aside from replacement.

[lurkingknight]

most brands don't lock the upper to the lower via the rear receiver pin. I believe only ICS split gearboxes lock at the rear pin and swivel open from the front pin.

All others slide straight over the gearbox and have a tongue that lock the upper into the lower at the back in varying methods. They are held in place by the front pin.

I doubt you can mix and match receivers as well, src is pretty offspec.

[sixb0nes]

Looks like a lot of the uppers have the male portion on the upper receiver, where SRC has it the opposite.

When I was referring to tabs, even SRC had them on their previous generation M4 uppers (at least from my investigation), they would wrap the gearbox on the rear and secure in place via the pin. I'm guessing this would help to keep it secure. Here is a video that shows what I think is a Gen2 with tabs that I don't have:



https://www.youtube.com/watch?v=GvSCFceTev0

Any suggestions otherwise, short of buying a new lower?

Originally Posted by lurkingknight

most brands don't lock the upper to the lower via the rear receiver pin. I believe only ICS split gearboxes lock at the rear pin and swivel open from the front pin.

All others slide straight over the gearbox and have a tongue that lock the upper into the lower at the back in varying methods. They are held in place by the front pin.

I doubt you can mix and match receivers as well, src is pretty offspec.