Tight BCM upper or loose Vltor MUR?

Alright, gents and ladies...I could use some advice from you human benchrests.

I have everything needed to build a precision upper(build parts listed below), but I'm facing a strange choice. I have a new BCM4 upper receiver and a new Vltor MUR, and a choice to make between the two.

1- The BCM4 has to be pushed on by hand, but isn't so tight as to need a rubber mallet introduced. Definitely doesn't just slide on, though.

2- The Vltor MUR slides on with a hint of wobble, though not a terrible amount. But the wobble is definitely there.

Which upper receiver would you use? (And if you could, please explain why.)

Parts list:
-BCM Comp
-Accu-Washer system
-Sionics 18" barrel (new version)
-Sionics gas block and gas tube
-Sionics 15" rail
-BCM BCG
-BCM Ambi Charging Handle
-BCM4 upper receiver (unused, but had on hand)
-VLTOR MUR-1A (bought for the build)

My two cents and thoughts: I bought the MUR specifically for the build because of its prevalence in being used for accuracy builds, thicker construction, etc. I had the BCM sitting around because I got a really good deal on it. Generally speaking, I would have thought the thick walled construction of the MUR would be an advantage over the standard construction of the BCM. I also understand that when you install a barrel nut it crushes the extension to the barrel, so it might be a moot point. But at this point I don't know where the advantages of one start to outweigh the other. Nor do I know when one has the advantage over the other when all things are not equal in regards to Barrel extension fit. So I'm hoping that the combined experience of the group leads to an obvious choice, given that there are a lot of uppers that have been built between the lot of us.

I have only built three precision uppers. All used the BCM4 upper receivers and the original 18" Sionics barrel, and all three shoot down to around 1.25 MOA using quality reloads not developed specific to the individual rifle.

I'd use the MUR, especially if planning shoot suppressed. The added stiffness for a heavier barrel and can(?) is only a benefit. I admit to ignorance in regards to how much the barrel nut installation crushes the receiver threads.

I built an upper with a WOA barrel and Vltor MUR. I had the same concerns, my barrel to upper fit was way looser than I wanted. I torqued it down with a Geissele rail and it shoots awesome. My last range trip I had two 5 shot groups at 100 yards that measured less than .7 moa. I don't think a super tight fit is as critical as the internet would have you believe. In my experience accuracy is for the most part in the quality of the barrel.

Quote:

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Originally Posted by Fatorangecat

I built an upper with a WOA barrel and Vltor MUR. I had the same concerns, my barrel to upper fit was way looser than I wanted. I torqued it down with a Geissele rail and it shoots awesome. My last range trip I had two 5 shot groups at 100 yards that measured less than .7 moa. I don't think a super tight fit is as critical as the internet would have you believe. In my experience accuracy is for the most part in the quality of the barrel.

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I have shot similar groups with a VLTOR MUR, Larue barrel, and Geissele rail. Wouldn't say the MUR wobbled, but it was nothing like a BCM.

Wobble is probably a bit of an overstatement, and is in comparison to needing to tap the barrel extension into the BCM4 upper receiver. I'll stay the course and use the MUR. Thanks for all the input.

Another consideration is the upper to lower fit. For my precision build I used a Bartlein 18" barrel in a BCM upper with BCM lower. The friction fit of the barrel to receiver was a little on the loose side. The fit of the upper to the lower was excessively loose for me. Remember that accuracy is based on consistency and eliminating as much inconsistency in semi-autos helps. My accuracy with that setup was around 1" 5 shot groups at 100yds. I changed out the upper receiver to a Hodge Defense upper and you basically need to heat up the receiver to get the barrel in. I then managed to get a lower receiver from Jim Hodge to complete the setup. With Geissele MK8 rail, DMR trigger, Leupold MK6 3-18 optic and 77gr FGMM its normally a 1/2 MOA gun now. There are various ways to tighten up the lower to upper fit, such as the accu-wedge, and there are many people who will actually add some blue loctite to the barrel extension to eliminate any play between the barrel to upper. Given your two options I would stick with the MUR and try some loctite on the extension. Good luck with the build.

I have used hardened stainless steel shim stock for make my barrel extension to upper fit a heat to install affair. The shim stock is available in 6" wide rolls from Amazon in various thicknesses. It's little finicky to install, but worth it in terms of consistency.

Never mind, bad idea retracted...

Quote:

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Originally Posted by tpe187

Another consideration is the upper to lower fit. For my precision build I used a Bartlein 18" barrel in a BCM upper with BCM lower. The friction fit of the barrel to receiver was a little on the loose side. The fit of the upper to the lower was excessively loose for me. Remember that accuracy is based on consistency and eliminating as much inconsistency in semi-autos helps. My accuracy with that setup was around 1" 5 shot groups at 100yds. I changed out the upper receiver to a Hodge Defense upper and you basically need to heat up the receiver to get the barrel in. I then managed to get a lower receiver from Jim Hodge to complete the setup. With Geissele MK8 rail, DMR trigger, Leupold MK6 3-18 optic and 77gr FGMM its normally a 1/2 MOA gun now. There are various ways to tighten up the lower to upper fit, such as the accu-wedge, and there are many people who will actually add some blue loctite to the barrel extension to eliminate any play between the barrel to upper. Given your two options I would stick with the MUR and try some loctite on the extension. Good luck with the build.

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Have you seen a change with that upper on other lowers, assuming the same trigger, stock, and grip configuration? I’m be run my BCM 410 SS upper on various lowers and it prints the same group no matter if it’s a tight fit or a loose fit. Curious to see what differences you have seen.


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I have a 20” Kreiger dcm profile installed in a bcm4 upper with a geissele rail. No problems so far. Negligible poi shift when on a bipod vs on the mag. I’d go for the tighter fit. That will help insure the bolt lugs having an even load in my opinion. Of course, my opinion is worth what you paid for it.

I appreciate everyone's advice. I went with the Vltor MUR, and will see if it ends up shooting as well or better than the build using the BCM4 upper. It'll be a good month or so before I can really wring it out.