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Discussion Starter #1
Hello, I am trying to sort out why my KMP m2 semi will never fully push the barrel forward when chambering a round unless I leave the top latch open, feed an individual round, and try multiple times.

I am reading the army troubleshooting manual... does this have something to do with the barrel buffer group spring adjustment? Is it just not having enough spring tension to fully move the barrel? When this happens with dummy rounds I can bump the barrel by hand and it will fix itself. Feeling a bit lost.
 

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When removing the spring, and sliding the bolt forward I am seeing that the cartridge extractor seems to hang when it makes contact with the Switch Extractor. It requires significant force to move the bolt forward from that point.
 

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is your barrel headspaced correctly? are you all the way tight? if so back it out a few turns and see if you lock up?

then buy a headspace gauge.
 

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It is headspaced correctly. I added a ridiculous amount of Hoppes 9 gun oil to various parts of the receiver internals and now the gun is feeding, chambering, and locking with my dummy ammo. The last 1-2 inches of travel has to drag the barrel buffer spring assembly forward and there seems to have been some friction in that movement. I'll give it another try at the range. I have heard that 20W motor oil can be used. Is motor oil better? Maybe it is more viscous.
 

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First off, who built the gun?

"Tightness" of the action can come from a variety of issues like the receiver "box" not square (i.e., not correctly riveted together), the breech lock cam too tight (it has to have some play in it) and frankly a new build can be tight and takes 50-200 live rounds to function properly. Keep in mind that new parkerizing is rough and until burnished, acts more like sandpaper than a smooth surface. (Remember the 1950-60's new car break-in period?) I've had several semi M2s built by various builders and many were tight. I hand cycle 200-300 times with a linked belt of dummy ammo first and then use full power loads (I load API with 230 gr 860 but it's not canister grade powder so this IS NOT a load I recommend to others. You need to work up your own loads) A good -but not excessive- lubing of the gun works best. Be sure to lube the bearing surface at the barrel support/barrel interface as it moves each cycle and a lot of weight (thus friction) from the heavy barrel rests there.

After the 50-200 rounds, API loaded to 220-225 gr 860 works the action just fine.

As to the box not being square, the M2 box is not near as forgiving as the 1919 receiver box about being out of alignment. It it critical that it be put together by someone who knows what they are doing. Knowing what you are doing when assembling the box is key.

The breech lock cam, per USGI 1953 rebuild manual, says:
"It should have a slight float and preferably clearance with the bottom plate of 0.001 to 0.008 inch, but may have clearance of 0.001 to 0.012 measured at forward end"
Forward end means measured by a feeler gauge inserted between the bottom plate and breech lock cam and between the installed cam's "legs" back to the breech lock cam bolt. Use automotive type feeler gauges. Make sure there isn't dirt and debris under the cam. I once received one back from assembly that was packed full of blast cabinet grit. Needless to say, it wouldn't "float."

Finally, remember to 1) check headspace with a good gauge and 2) Feed the belt into the gun with the double loop end of the belt entering the gun. The single loop end of the belt should be the on the tail end of the belt being fed into the gun. Headspace too tight and the gun won't function very well; Headspace too loose and you'll blow the gun (and quite possibly yourself) up!!!.

There are other possibilities but these are the first likely culprits.
 

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Discussion Starter #6
OREO thank you so much for the detailed information. The Breech lock cam clearance is .009". I am using a headspace gauge before firing. Internal condition leads me to believe that most components of the the M2 were unfired before my purchase. According to KMP, it is their sideplate, but not their build. I believe the parkerization on multiple moving components is causing some friction. Is there a specific type of oil I should be using? I've been using Hoppe's 9 gun oil but I've also been told that you can use 20W motor oil.

As for the ammunition, I'm using my own 225 grain WC872 loads with military ball bullets.
 

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Motor oil works just fine. I personally use 5-30 synthetic but just about any motor oil should fine. The brownings like to run wet. I use period correct oil can, open the top and squirt it down the inside of each plate pull the action back and get the barrel support and cycle a few times. If shooting a lot repeat after a few belts. I do the same with the 30cals.
Greg
 

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Most any type of lubricating oil works. Lots of folks use motor oil. It's cheap, relatively speaking. I've used an ATF mix before with decent results. Now I personally use some type of GI oil (LSA or WWII medium weight gun oil but the latter is getting tougher to get) as the military put a lot of engineering into these lubricants/preservatives for their intended use.

Using a lot of lube for recreational use works fine (except for the oil spray on your safety glasses, etc.) but oil seems to attract dust and when mixed together forms a pretty good lapping compound which can wear parts, so personally I'm always a little leery of too much oil.

In my opinion, CLP is slightly better that WD40 (and a LOT more smelly) but not really very good as a lubricant. Seriously, how can a lubricant also be a cleaner (i.e., solvent)? Ah, but then perhaps I'm too old fashioned and don't see the beauty of CLP!!!
 
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