Building and fielding my AR15 SPR (and a pic)

JasonMPDJasonMPD Senior MemberPosts: 6,104 Senior Member
My AR15 SPR (special purpose rifle) build has been a great trek into the realm of precise, intermediate range AR15 rifles. It's been a learning experience with what it takes to build an AR15 purpose-built for precision. An average AR15 can be slapped together and will shoot the military standard 3MOA-or-less with SS109 without any effort. Quite literally.

Assembling a SPR is different. And I found it different in some key areas:

The barrel. It's no mystery that a quality barrel does a precision rifle make. It's arguably the most critical component. I find this extra critical for a SPR. For a bolt action rifle there are only two dimensionally critical areas: the bore and the thread tenon. And as long as the barrel maker does their part, a simple indication on the bore will yield a quality thread tenon.

The SPR adds some dimensions. The barrel thread tenon accepts a barrel extension which inserts into the receiver upper barrel port. There's a huge potential for tolerance stacking if the dimensions are crud.

Next is the gas block journal. It's dimensionally critical because the gas block clamps or pins around the circumference of the barrel. If it was simply welded or brazed at the 12 o'clock position it's nothing. Tolerance stacking here can put lateral forced on the barrel. Additionally to this is the gas port. Some are drilled and some are finish reamed. The latter uniforms the port and creates consistent gas flow.

Then there is barrel contour. Heavier is better, but if it's fielded heavier sucks. The happy medium? Not sure. I think if an 18" barrel could be had in the .850" barrel size common to AR10 barrels you would be in the money. I've begun to search. It's not a common barrel weight for 223/5.56. My Mega Arms barrel starts at about .850" behind the gas block journal and is .740" to the muzzle threads. That's Mk12 Mod-X dimensions, but I want heavier.

Next is the gas system position. My barrel is 18" long. I opted for a rifle length gas system for one primary reason and that is to get the bullet as far down the barrel as possible before the gasses enter the gas tube. In theory, this lessens the effect of slight bolt carrier movement on the system before bullet exit from the muzzle. I didn't try a mid length and carbine length gas system prior to this one, but it eased my mind.

Upper/Lower fitment. My Rainier Arms forged mil-spec lower came with an upper receiver tension screw as standard. It presses up against the upper receiver rear takedown pin area and stops movement between the upper and lower. I've shot it with and without screw tension. It made my shooting almost 1/3-MOA better. This fitment is important.

Trigger. Do NOT slouch here. I'm not saying 1 MOA can't be done with a mil-spec trigger, but my Geissele SSA trigger is infinitely more consistent than a mil-spec trigger can dream of being. Pick a good one.

Grip. Man, did I try a bunch. Standard, Magpul MOE, Ergo, Stark...blah blah. The Magpul "K" series grip won out eventually. It's something like 17-degrees more vertical than standard and it made the world to me. Your mileage will vary, but consider this as an option.

Scope mounts. Dispense with dual scope rings and go with a one piece mount. I haven't gone that route yet, but I will as soon as I find a superb brand second hand. A one piece mount dispenses with tolerance stacking across the upper pic-rail and prevents misaligned rings torquing on the scope. Secondly the one piece mount also distributes clamping forces along a longer section of the rail and over-torquing is less likely to damage the upper rail with repeat tightening.

These are just a few of the things I've come across. Having a bolt specifically headspaced to a barrel is a plus too.

Here she is in her current form. She's a solid 1-MOA gun. Hopefully better soon.

imagejpeg_0_1.jpg
“There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves.” – Will Rogers

Replies

  • timctimc Senior Member Posts: 6,684 Senior Member
    SWEET!
    timc - formerly known as timc on the last G&A forum and timc on the G&A forum before that and the G&A forum before that.....
    AKA: Former Founding Member
  • Big ChiefBig Chief Senior Member Posts: 32,995 Senior Member
    So what, I'll bet I can tear a 33 gal trash can to pieces @ 100 yards with my iron sighted Ruger 556 and enough 30 round mags..............:tooth: :whip2: :whip2:

    Interesting, thanks for posting besides.. I have my ONE and Only AR


    ps...don't hold me to that statement :guns::guns: :jester:
    It's only true if it's on this forum where opinions are facts and facts are opinions
    Words of wisdom from Big Chief: Flush twice, it's a long way to the Mess Hall
    I'd rather have my sister work in a whorehouse than own another Taurus!
  • JasonMPDJasonMPD Senior Member Posts: 6,104 Senior Member
    cpj wrote: »
    Or, just buy the DMPS heavy barreled upper. :tooth:
    Mine shot honest .25 groups. With hand loads. PMC ammo shot at best 1.5" groups.

    Well, I like to build and I don't have a 30 year old R&D department. :tooth:

    And in don't want a 24" barrel that's .900" at the muzzle.

    ETA: I looked up that upper and I see it has a 16" version too. Still heavy as hell with a barrel of that contour.
    “There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves.” – Will Rogers
  • JermanatorJermanator Senior Member Posts: 15,230 Senior Member
    Big Chief wrote: »
    So what, I'll bet I can tear a 33 gal trash can to pieces @ 100 yards with my iron sighted Ruger 556 and enough 30 round mags..............:tooth: :whip2: :whip2:

    Interesting, thanks for posting.
    There you go Chief, I fixed it for you.
  • Farm Boy DeuceFarm Boy Deuce Senior Member Posts: 6,083 Senior Member
    I like the looks of that rifle. I am sure you will get better than 1 MOA, when you do some more load work.

    My DPMS heavy barrel has only done 1/2 MOA at two hundred so far.
    I am afraid we forget sometime that the basic and simple things brings us the most pleasure.
    Dad 5-31-13
  • JasonMPDJasonMPD Senior Member Posts: 6,104 Senior Member
    I like the looks of that rifle. I am sure you will get better than 1 MOA, when you do some more load work.

    My DPMS heavy barrel has only done 1/2 MOA at two hundred so far.

    I've actually thought about getting the DPMS Mini SASS barrel by itself. 18", 1:8, heavy contour, fluted. Just need to turn down the front 2.5" of barrel to .770" for the RSTA suppressor mount. Slightly heavier than I want in a barrel, but I bet it shoots.
    “There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves.” – Will Rogers
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