Hey Guys, first post on the board since I finally decided I'm going to build an 18" SPR. I tossed around the idea of a MK12 then decided I'd rather go the custom route with some newer components, particularly the Hand Guard.


The main goal for the rifle is not necessarily the best group, although accuracy is important its more to be able to get out to 500-600 yards with a battle rifle that will last and be durable. That said, I ended up going with the:

Ballistic Advantage 556 Performance Series 18" SPR Barrel (Ops 12 ) - (CMV, QPQ, NiBx chamber)

I have a couple items I'd like some advice on....

With the 18" barrel and the Rifle length gas system does anyone know how much space I should leave exposed after adding a rail, allowing enough room for the Ops Inc collar?

I'm still not all the way sure if I am going to add the Ops Inc collar or just go with a 15" or 18" Midwest Industries SP hand-guard which allows for the suppressor to be installed under the rail.

I'm also a little confused about the stock. Should I wait until I can get the Viltor A5 or will it not harm anything to run this barrel with a Spikes Milspec Carbine buffer Kit? I picked up a Magpul ACS that should work with either since its Milspec but not sure about the carbine buffer.

Thanks much!

I would stick with the 15" handguard. The OPS collar looks like it's about 3/4" in front of the gas block. I'm pretty sure an 18" handguard would be past that.

As far as the buffer, you should be fine using what you have.

Ok, great. Appreciate the feedback.

If anyone has any experience or comments on the barrel I finally went with I would like to hear about it! That was the toughest decision of this build!

I'd stick with the 15" rail too. I have a VLTOR Emod on my 18" barrel SPR. Works good.

A 15" rail works perfectly with an 18" barrel and AEM5.

When you get the Vltor A5, I'd suggest going as heavy as possible on the buffer.

Ok, thanks for the tip on the buffer weight for the Viltor.

Sounds like the 15" rail is the way to go which is what I was thinking but wasn't sure.

I'm liking the Midwest rails the most so far and probably pick up the Gen 3 within the next couple of weeks.

Thanks again.

I have the BA 18in SPR Ops barrel with a 15in rail. It is a great grouping barrel with the expensive/ good production ammo. The groups are identical to my 20in Larue Stealth barrel.

Attached File

Awesome, nice Rifle.

I have a few follow up questions from the above comments.

Sounds like the AEM5 will be the way to go, however, for the Collar will I be able to use the "SPR Collar" or will need to maybe go with one of the smaller ones like the M4 sized one?

Also, in regards to the Viltor A5, I'll need a new buffer spring and buffer. Sounds like I'll want to use the H3 will I need to use a Rifle size spring with that or is there maybe a combo deal that I should be looking for to purchase the buffer with the spring?

Also, sorry for the late replies is there a way to turn on email notifications for the new posts?

Thanks again guys!

If your barrel has the SPR/Mk12 profile, you need the SPR collar. If your barrel has a "Govt" profile with a .700" x .700" step down at the end, you need the AEM5 M4 collar. If your barrel is neither, you will have to have a custom collar made.

The A5 is available as a kit with the receiver extension, A5H2 buffer, action spring, end plate and castle nut. Bravo Company also sells a kit with an A5H0 buffer. I like to use an A5H4, Colt action spring and QD end plate so I buy my parts separately. An A5H0 will function best in a rifle that has a smaller gas port and an A5H4 will only function in a rifle that has a larger gas port and then there's everything in between. I use an A5H4 in all of my builds and if it doesn't lock the bolt back on the last round, I open the gas port until it does (after confirming there aren't any problems elsewhere in the system). It doesn't have to be a huge gas port to work with the A5H4. For example, an A5H4 works with a .062" gas port on a 14.5" barrel. You can tune the weight of your buffer to your existing gas port by adding or removing tungsten weights. Making an A5H4 from a lighter buffer is a little more difficult. The last weight on al the A5 buffers is slightly different with a blind hole in the end that the spring rests inside. To go from a tungsten rear weight to a steel rear weight or the other way around, you have to drill that hole.

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Ok, this clarified the confusion I was having. I seen the Viltor A5 receiver on sale on Primary Arms without the spring and buffer and wasn't sure what I should go with using an 18" SPR MK12 Profile barrel.

Based on the great info you provided here I think I'll measure the gas port and base the buffer weight on that.

I wasn't aware of the subscription I'll take a look at that since this site seems to be invaluable when working on these projects. I hope to report back soon with some pics and status of the final build.

Thanks again!

I suggest getting the A5H4 and then if it doesn't work and you don't want to make your gas port larger, you can make it lighter by swapping weights with one of your other buffers. You'd also be making your H buffers H2s or H2s H3s as needed. I test mine using the weakest ammo I'll ever use and make sure it locks open on an empty mag.

Great advice which sounds like this is more then likely what I will end up doing. I'll probably start with one of the kits which will give me at least an initial test and see where I am at. I'll probably go with the Viltor kit instead of the BC since it comes with the H2 and not the H0. I hope to be finishing this up within the next few weeks and reporting back. I just need the rail, pretty dead set on the Midwest 15" G3M SP model and the receiver kit. I bought the Spikes kit early on which I'm sure I'll find a use for!

At this point I'm dying to find out how the Barrel will do! Ballistic Advantage 556 Performance Series 18 SPR Barrel (Ops 12) I'm hoping I did not make a mistake going with the CMV QPQ instead of the stainless. I figured since they are guaranteeing sub MOA it better should well! I inspected it pretty closely and it looks spot on.

Thanks again!