A few questions on MP40 build

[XboxMeister]

I have an MP40 parts kit and a 80% aluminum tube receiver I bought from someone a while back and I am in the process of finishing the receiver to make a non-firing display gun out of it. I've machined some of the slots into the receiver tube but have a couple questions:

1) The receiver lock knob in front of the trigger controls a small rectangular catch that appears to turn so that the catch locks the receiver down. During my test fits, this appears to be a pretty tight fit to get the catch to get high enough so that I can have the "wings" of the catch turn inside the receiver and lock it down. Is that expected or am I thinking about this wrong in some way?

2) I cut off the original receiver fragment from the trunnion (barrel bushing). It was attached with a pin through the receiver/trunnion and also crimped in several locations around the receiver into indentations in the trunnion. I've read these were also brazed but I saw no sign of that. When I tried to dry fit the receiver tube on the trunnion, I found the inner diameter of the tube was a bit too small. The trunnion is not completely round but rather is about 1.45" top to bottom and 1.38" side to side (as measured with my calipers). The inner diameter of the tube is about 1.38" So I am assuming I need to grind out some material on top and bottom inside the tube to allow the trunnion to fit (taking out about 0.035" on both the top/bottom inner faces to the depth of the trunnion). Does that sound correct? The receiver plans I have seen make no mention of this but it seems like the only way it will work. The top and bottom of the tube are the thickest as they have wide ribs so I think it will still be quite strong.

3) As I assume I will need to crimp the tube onto the trunnion down the road, any suggestions on how to best do this? Hydraulic press perhaps with a hardened steel tool of the right shape? Is there a recommended way to do this?

Thanks in advance for any help.

 

XM

[DINK]

Can't help with the trunnion stuff, but the receiver catch doesn't pivot inside the receiver to hold it in place- it just rises up into the hole and keeps the receiver from turning. The knurled knob is pulled down and rotated to keep it in the "down" position for field-stripping. When the upper receiver is clean and back in place, the knob is rotated again to release it and it snaps back up into the hole to lock the receiver in place. The front of the receiver is held in place by a small "tongue" of steel on the front of the lower receiver assembly. This slides into a slot on the rear of the magazine well assembly and hold the front of the lower in place. Between the "socket" in the rear and the little tongue in the front, the two halves are held together. The catch just keeps the upper from rotating.

 

HTH

[XboxMeister]

Great, thanks DINK for the info. Appreciated. That makes sense. I was looking at it again last night and realized there is just no way it could get high enough and the knob won't even turn until it is pulled down even lower. And I suspect it would interfere with the bolt sliding inside the tube if it could.

 

XM

Can't help with the trunnion stuff, but the receiver catch doesn't pivot inside the receiver to hold it in place- it just rises up into the hole and keeps the receiver from turning. The knurled knob is pulled down and rotated to keep it in the "down" position for field-stripping. When the upper receiver is clean and back in place, the knob is rotated again to release it and it snaps back up into the hole to lock the receiver in place. The front of the receiver is held in place by a small "tongue" of steel on the front of the lower receiver assembly. This slides into a slot on the rear of the magazine well assembly and hold the front of the lower in place. Between the "socket" in the rear and the little tongue in the front, the two halves are held together. The catch just keeps the upper from rotating.

 

HTH

[Black River Militaria CII]

You will want to locate and cut the hole in the bottom of the receiver for the flat sided takedown stud very carefully. If it is too wide, the flat sided stud will be "loose"in the slot and there can be rotational slop in the tube. Even if the tube is tight at the rear where it bayonets into the end fitting of the lower, if the slot is too wide, the tube can still rotate slightly in either direction when holding the magwell in a firing position. Many live MP40s suffer from this to a greater or lesser degree from many takedowns and wear in the width of the slot from firing use, etc.

[XboxMeister]

You will want to locate and cut the hole in the bottom of the receiver for the flat sided takedown stud very carefully. If it is too wide, the flat sided stud will be "loose"in the slot and there can be rotational slop in the tube. Even if the tube is tight at the rear where it bayonets into the end fitting of the lower, if the slot is too wide, the tube can still rotate slightly in either direction when holding the magwell in a firing position. Many live MP40s suffer from this to a greater or lesser degree from many takedowns and wear in the width of the slot from firing use, etc.

Good feedback. I will be careful about this. Right now it seems to pretty snug and don't see any rotation happening when I twist on the tube but can see how that could develop over time.

 

XM

[TSMGguy]

Yup, every MP-40 has a little marring around the hole for the take down stud in the receiver. It's from where people thought that the receiver was aligned, turned the take down stud to get it to snap into place, and then had to rotate the receiver a little one way or the other to get the stud to lock into the receiver.

 

I suspect that my BNZ-41 MP-40 was issued to a soldier who carried it from new until it was captured in Normandy. There's evidence that he always missed aligning the receiver by the same amount every time he field stripped the gun. He'd then turn the receiver to lock it, which, over time, left a distinctive little gouge in the steel. I try to be a little more careful.

[XboxMeister]

I bet. I could see that happening pretty easily.

 

Anyone have any thoughts on the trunnion/receiver fit question? I'm getting close to starting to grind out the extra material but before I take the plunge want to make sure I'm not missing anything.

 

thx

 

XM

[Toaster]

Rather than grinding the tube, is there anyway you can get the trunnion OD turned down on a lathe? That would ideally help keep things centered and snug.

[XboxMeister]

Rather than grinding the tube, is there anyway you can get the trunnion OD turned down on a lathe? That would ideally help keep things centered and snug.

I suppose it is possible but I'd prefer not to modify the original part. I'd probably go buy one of the trunnions I see out there that seem to be more round (perhaps different years?). Do you (or anyone) know how the dimensions of the trunnions vary (not tolerances but in how they were made, different designs, ...)?

 

thx