One lower, two uppers, three questions


October 14, 2012, 02:55 PM
173436173437So here goes, I've got an LMT Defender 2000 lower, an LMT CQB16 14.5" upper pinned to 16.25", and an AAC 16" upper in 300BLK. All purchased separately. The LMT upper and lower are so tight I have to use tools to get them apart. I understand a break in process but this just seems a little abnormal. It shouldn't be this difficult to change my uppers. Have any of you seen this problem in similarly machined products? My next question has to do with barrel break in processes, do any of you follow them with your non precision weapons? Is there an "AR barrel break in process" I don't know about? And finally, is there glass out there that I could swap between uppers without losing too much accuracy, that would be shooter friendly for both calibers? Or should I just fork out for two scopes? Thoughts? Any advice, or experiences much appreciated! -Brandon

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October 14, 2012, 04:18 PM
Hello Collector0311, I bought a Red X 300 Black Out upper and it is a good fit to my Anderson Mfg lower and Aero Precision Lower, but tighter on the Aero. I feel a tight fit is better than a looser one and should wear in over time. Resist the urge to start sanding on mating surfaces. As to barrel break in periods I asked that exact question and the opinions on many forums was it is a waste of time. I thoroughly cleaned my barrel, leaving a light coat of CLP in place before shooting the first 20 rounds of Remington factory 115gr ammo through her. Followed by a very thorough cleaning and repeat for the next 20 rounds. My groups are improving with practice and recently mounted optic so it looks like this was sufficient. As to switching optics? I am no great scope guru but feel that a scope is usually mounted then zeroed to one receiver barrel combination and then left there, hoping and praying the zero hold after being bumped and jarred during normal handling. If you buy a quick disconnect mount and move your scope from upper 1 to upper 2 you will still have to rezero the scope. I feel these quick disconnects are meant to get a damaged optic out of the way of your back up iron sights in an emergency. If you do decide to fork over the money for two scopes check out Midway USA they have a sale on optics. Good Luck.

October 14, 2012, 05:08 PM
Thanks for your quick and thorough reply photonSD. I am no stranger to having to get pieces broken in to attain a good mating between receivers. However, in this case I find myself attaching the forward pin and rocking the receivers back to meet and have to clamp about a sixteenth of an inch gap between the two, then mallet the other pin in. Getting them apart is just as much of a task. Now I'm hesitant to go shoot because if it IS a factory defect I want to send it back unfired.
Thanks also for your take on break in techniques, I figured I'd get a million opinions ranging from don't clean it until after 10,000 rounds to clean it after every round for the first 100. I think I'll just shoot a box and do a thorough cleaning and repeat for a few boxes.
I figured I would need two sets of glass but ya never know these days with some of the cooler stuff that's hitting the shelves. But I would like to go with top of the line equipment across the board and was looking at an ACOG4x for my LMT upper but haven't heard much in the way of good glass with BDC set ups for 300blk...

October 14, 2012, 05:37 PM
It's not unusual for uppers and lowers to need a little extra persuasion to fit together, but clamps and mallets sounds like a bit much. If it's not too late, I would return the 300BLK upper and try again from a different manufacturer, or if you really like the barrel and handguard, try buying a different stripped upper to achieve a better fit with your lower.

October 14, 2012, 10:38 PM
The thing is its my LMT receivers with the bad fit. My AAC 300 fits great

Well Now
October 14, 2012, 10:46 PM
I have a Spikes lower and a Mega upper that are tight. At one point I was using a wood golf tee to push the front pin out. After playing with it a bit pushing the pins in and out, there seems to be a slight improvement.

October 14, 2012, 11:58 PM
Well, the logic still stands. If you aren't attached to the offending upper, sell it off and try again. If you like the barrel enough to want to keep it, try a different stripped upper and if the fit is good then swap out the parts. It shouldn't take a major operation to assemble your rifle.

Well Now
October 15, 2012, 07:31 AM
What's interesting with my situation, is that I have three other uppers and lowers of various manufacturers and when interchanged with the "offending" upper and lower, they all work fine. Maybe that particular set just doesn't like each other and want a divorce.

October 15, 2012, 07:49 AM
Shoot the tight pair, they well loosen up after a few hundred rounds.

The rear take down lug on the upper should be oval shape. It's a .250 diameter hole and it's machined oblong about .010-.015".

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