Talk Me Out Of A MK18 Upper

[Koshinn]

If you want, hit me up via pm and you can shoot my 10.5". It's not a mk18 as it has a NSR instead of a RIS II, plus an additional 0.2", but it's fairly close.

I'm in Vegas. You just provide ammo. But not Wartak.

[JG007]

What about a 12.5?


And my buddy wants a mk18, I went 11.5 with A5 and was just thinking that both should be reliable but they go about it differently, is this correct?

[Bluto]

Taken from a conversation with the CEO of BCM on this thread: https://www.m4carbine.net/showthread...0-5-quot-upper

Q: Why BCM chose the 11.5" SBR over the 10.5"

I get this question a lot. It is a good one. When we spec our program we can build anything we would like, so we sat down and looked at the pros and cons.

First Statement: I know a good 10.5" SBR can run very well. I personally own a semi MK18 type and it runs great. No problems.

A: Dwell time.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).

The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.

[Turnkey11]

LMT 10.5 upper with a M4 RAS is the way to go. Runs out of the box, more than I could say for the upper I received from DD...

[TehLlama]

LMT 10.5 upper with a M4 RAS is the way to go. Runs out of the box, more than I could say for the upper I received from DD...

Agreed, and they retain resale value almost indefinitely because it's such a known solid setup.


For somebody truly in doubt, the BCM 11.5" upper with the MK18RIS-II is still a truly excellent upper to build from, with all the feature set most people want from a Mk18. Similarly, an 11.5" with the 10.75" URX (3.1 or upcoming 4); or the truly sublime KMR handguard 11.5" KMR setup are all really solid starting points. To get an Mk18 without at least evaluating each of those options is a setup for winding up having one of each just out of buyer's remorse.

I personally run a modified Mk18 setup myself (10.4" barrel with URX3 8.0" handguard that accommodates an OPS 14th can - running a Rosch SLM-2E w/ VTAC L4-Mini and LDI DBAL-I2 in front of an Aimpoint T1; sitting on my needlessly gorgeous Noveske Gen2 Chainsaw SBR lower - G&R LPK, GSSA, VLTOR A5, Magpul MIAD/ACS), so consider that a qualifier on my nearly universal recommendation to others to get an 11.5" setup. My upper originated as a communication error with ADCO, but if I run my M4S can on it, it's a carbine that fits into a tennis racket case in ready-to-fire setup (assembled, loaded magazine in, suppressor mounted), which is seriously a cool setup... but functionally that kit does very little that a well sorted 11.5" setup with a more up to date QD suppressor couldn't do.

The OAL gain is small, going to 10.5", you'll only be able to really notice any difference is that the mass of a can moves a full inch farther away with an 11.5" deal, which becomes relevant after you've larded the upper up with weaponlight, control accessories, sights, and maybe IR laser/illuminator capability on top of running dangling a ~1lb can and muzzle device at the end... but that's predicated on the assumption that you'll be using it as a double stamp setup. If it's not a double stamp, then 11.5" all day every day.

For the OP, you've already got a really phenomenal setup - if that isn't doing all the things you want, then start really evaluating what about isn't the exact flavor of unicorn you want - might be as simple as a different handguard or muzzle device, or even going to a smaller optic - because from where I'm sitting that setup is pretty sorted.

[HKGuns]

Each of my DD MK18 uppers have been 100% out of the box. I don't see a lot of complaints about them despite what the LMT fans are claiming.

[BigLarge]

I have ~1K rnds through my factory built DD MK18. It has been flawless, albeit a bit over gassed. For most people wanting an SBR, IMO, 11.5 is the better choice. I'm dealing with extreme CQB issues so that 1" does make a difference for me (I also just like semi-authentic military weaponry).

[superstratjunky]

Ok I will, don't do it. Buy another one. There problem solved.

[702Warfighter]

Agreed, and they retain resale value almost indefinitely because it's such a known solid setup.


For somebody truly in doubt, the BCM 11.5" upper with the MK18RIS-II is still a truly excellent upper to build from, with all the feature set most people want from a Mk18. Similarly, an 11.5" with the 10.75" URX (3.1 or upcoming 4); or the truly sublime KMR handguard 11.5" KMR setup are all really solid starting points. To get an Mk18 without at least evaluating each of those options is a setup for winding up having one of each just out of buyer's remorse.

I personally run a modified Mk18 setup myself (10.4" barrel with URX3 8.0" handguard that accommodates an OPS 14th can - running a Rosch SLM-2E w/ VTAC L4-Mini and LDI DBAL-I2 in front of an Aimpoint T1; sitting on my needlessly gorgeous Noveske Gen2 Chainsaw SBR lower - G&R LPK, GSSA, VLTOR A5, Magpul MIAD/ACS), so consider that a qualifier on my nearly universal recommendation to others to get an 11.5" setup. My upper originated as a communication error with ADCO, but if I run my M4S can on it, it's a carbine that fits into a tennis racket case in ready-to-fire setup (assembled, loaded magazine in, suppressor mounted), which is seriously a cool setup... but functionally that kit does very little that a well sorted 11.5" setup with a more up to date QD suppressor couldn't do.

The OAL gain is small, going to 10.5", you'll only be able to really notice any difference is that the mass of a can moves a full inch farther away with an 11.5" deal, which becomes relevant after you've larded the upper up with weaponlight, control accessories, sights, and maybe IR laser/illuminator capability on top of running dangling a ~1lb can and muzzle device at the end... but that's predicated on the assumption that you'll be using it as a double stamp setup. If it's not a double stamp, then 11.5" all day every day.

For the OP, you've already got a really phenomenal setup - if that isn't doing all the things you want, then start really evaluating what about isn't the exact flavor of unicorn you want - might be as simple as a different handguard or muzzle device, or even going to a smaller optic - because from where I'm sitting that setup is pretty sorted.

Thank you for your detailed explanation. My 11.5 won't be suppressed anytime soon so that part doesn't really play a part. I've heard mixed statements how some people say that an extra inch won't matter where some say it does. I like the setup I have and I prefer the fsb. I guess my curiosity with the DD mk18 is the fact that I don't own it lol. I have had a ris ii rail in the past and I prefer the shape of the omega I have right now better. I just wanted to see opinions out there on the 10.3. Even then some of what people are saying on here is that it does run good and that it doesn't. I still have a lot of thinking to do. It's hard going and dumping 1k on an upper just to not end up liking it or having problems with it. I appreciate you helping though.

[TXBK]

I had zero reliability issues with my dd 10.3 with the adjustable gas block wide open. The reason that I installed an AGB to begin with was because I expected it to have a gas port on the large side, and I knew that I would be running 5.56 ammo through it. Initially, recoil was very sharp to the point of being rough. Adjusting the gas block smoothed it out immensely. My friend wanted that gas block to fail so he could laugh, but it was I who had the last laugh. But, still, I enjoy shooting the 11.5 more.