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View Full Version : Precision semi-auto off an AR15 possible?



Booletsnotreactwell
07-01-2015, 12:29 AM
Thought I'd post this here while it's gonna require some AR knowledge to fully understand the big picture should still be understandable, it's still a rifle after all. While bolt guns seem to dominate in the precision rifle field, there are semi-autos that can push that envelope too, while they'll never be as good that's the trade off for a semi-auto.

I'd like to start by asking, what would be the best possible platform to build a precision semi-auto off of? My research so far has indicated that the AR15 platform and the mechanism in it is very capable and probably in the top 5 most accuracy capable semi-auto mechanisms around. If somebody knows different I'd like to know and maybe we'll go from there.

This build will be in .223 Rem or maybe 556 but so far I've read that .223 chambers hold tighter tolerances than 556 ones. While this may be seen as an intermediate caliber this is simply a proof of concept, if this build turns out good there isn't anything stopping me from upping up to .308, the design and mechanisms are the same. The only difference being that .308 will likely give better performance in the real world at longer distances. So I will hold this .223 to the same standard I would hold an equally accurate AR in chambered in .308.


The thing is, compared to a bolt gun, not much seems to be able to be done with this platform when it comes to accurizing it. There isn't any bedding of the action or bolt, lapping/polishing that I know of. I've read that some people have done such thing, put glass bedding between the two receiver halves or somehow within the hole in the receiver where the barrel extension sits (barrel sits in barrel extension) but I fail to understand how this would even affect accuracy with this design.



Let's forget fancy grips, triggers, stocks, etc. While these things do "improve" accuracy technically that's all the shooter and not the platform. Focusing simply on the few things that actually impart on the repeatability of the shot.

So right now the bone stock platform is a Colt AR15A4 which is your standard 20" full size, full length rifle gas, fixed stock AR15.

Barrel - I'm thinking 20 or 24" stainless steel match with a 1/7 twist with .223 chamber, likely Krieger with gas port hole drilled into land of riffling.

Gas Block - low profile adjustable gas block that's pinned to the barrel with tapper pins.

Bolt - While me and quite a few others have doubted thing such as "match bolts" for this type of gun over stock bolts, the tightest tolerance deviation can't be a bad thing. Some manufactures apparently make bolts of tighter "match grade" and some offer something custom like Shilen bolts that are matted/headspaced specifically for a barrel they sell with the bolt. Either way this will be a stock parkerzied bolt without any special coatings like chrome that can be unevenly applied and affect tolerances.


Upper Receiver - The guts, where the barrel/barrel extension mats to the rest of action. Going to use either a standard milled billet upper receiver or possibly a one piece monolithic depending on what answers I get here for the following option.

Free float - Many options exist for free floating the barrel on an AR15. Typical tube style handguards attach to the barrel nut which is torquing the barrel in place into the receiver instead of the normal handguards which simply rest on the barrel. While these free float and keep inconsistency off the barrel for most shooters, they do however put whatever stresses from things attached to the free floating handguard (bipod/sling) onto the barrel nut holding the barrel in place. Testing has showed that this flexing can cause misalignment by a few thousandths of an inch with the bolt and barrel extension which is where the bolt locks up. The other option for floating and choice of upper receiver is a monolithic type that extends as one piece over the barrel thus free floating it. The barrel is still floated however the mechanism to float it places the stresses on the upper receiver instead of directly on the nut that holds the barrel torqued into the receiver.



So there you have it, barrel, bolt, upper receiver and free float. To my knowledge that's all you can do on an AR15 platform in regards to accurizing it. I've looked at military specific builds where they accurize the AR15 such as the SAM-R, SDM-R and MK12 SPR. They only tackle those four things in the design and those are the only four things I can see that have an affect on the bullet and repeatability, the rest seems to be all on the shooter. If there is something I missed or I'm not understanding about the design feel free to let me know. Are there other angles to this when it comes to accurizing a semi-auto or building a precision semi-auto?

VooDoo
07-01-2015, 10:19 AM
Looks like this is what you might be looking at - if you can order it : http://www.coltcanada.com/iur.html
or get just the upper and re-barrel if a monolothic upper is that important for you.

Personally, as bench shooters we don't put enough of a load on the rails to affect accuracy. I tend to baby my rifles :)

Booletsnotreactwell
07-02-2015, 07:15 AM
Looks like this is what you might be looking at - if you can order it : http://www.coltcanada.com/iur.html
or get just the upper and re-barrel if a monolothic upper is that important for you.

Personally, as bench shooters we don't put enough of a load on the rails to affect accuracy. I tend to baby my rifles :)

We'll this is why I'm asking precision rifle shooters. You are all familiar with how the free floating function of your rifle works, I'm not, I just know the two options available for this type of firearm. If high end 1/4 MOA precision bolt guns have the forearm floated off the action/receiver then there is no reason to go for a monolithic receiver, however from what I can see a bolt gun and a barrel that's free floated looks a lot like those one piece IUR your linked. One piece AR15 receivers are basically the same except upside down, same concept.

I'd like to hear from guys that understand the mechanisms in precision actions such as those found in peoples rifles here and more specifically precision semi-automatic actions.

Defenstrator
07-03-2015, 05:09 PM
Looks like this is what you might be looking at - if you can order it : http://www.coltcanada.com/iur.html
or get just the upper and re-barrel if a monolothic upper is that important for you.
Man, now I wan't one of those.

50 B.M.G.
07-03-2015, 05:28 PM
From a person who manufacturers high end precision AR platforms.
20" barrel is more than adequate for 223, ALL of the powder is burned in the first 13 inches of the barrel anyway, so unless you plan to run iron sites, which kinda defeats the precision part, there is no advantage.

Adjustable gas blocks are a bad idea, they never work well of for long.
Pinned on gas blocks only means it will be impossible for perfect gas port and vertical alignment. On a barrel precisely machined to fit the gas block, set screws are a better plan.

Contrary to what many internet experts claim, how tightly the upper and lower mate IS important. If the upper is a few thou off from shot to shot how can the rifle ever be accurate.

Matchgrade bolts are a sales gimmick. Using the bolt head that is to be used with the specific barrel for headspacing is more important.
That being said using a decent quality bolt assembly is a great plan.

Select the CORRECT twist rate for the projectile you intend to shoot. many seem to think that the 7 twist is somehow magical. It is not.

A free floating handguard will always provide better accuracy than 1 that is held in place by the Delta ring. The jury is out currently on monolithic uppers. It seems to make sense that they would be an improvement over separate parts, but so far I have yet to see any concrete evidence supporting this.

We have ARs that shoot .5 or better, so the AR15 can be quite accurate. I doubt any semi will ever eclipse the accuracy a bolt rifle can produce, but they can do very well when properly built.

Booletsnotreactwell
07-04-2015, 09:13 AM
Pinned on gas blocks only means it will be impossible for perfect gas port and vertical alignment.

how tightly the upper and lower mate IS important. If the upper is a few thou off from shot to shot how can the rifle ever be accurate.

Select the CORRECT twist rate for the projectile you intend to shoot. many seem to think that the 7 twist is somehow magical. It is not.


I'd really like you to explain this, mainly I don't understand why that is so and I'd like to learn. The optic is held on the upper receiver, the bolt and the barrel are too. The lower is your stock and your fire control group. The way I'm seeing it (could be wrong) is even if there is some movement, the barrel and scope are going to reflect this movement as they are part of the upper. The play between the upper and lower (being a few thou off) seems like it would fall within the margin that most high end scopes are parallax free.


Second, what about lapping the face of the receiver? Is that another gimmick? Lapping the bolt too?


And yes correct twist rate is going to be selected, mainly going to shoot .223 hornady 75gr BTHP.

50 B.M.G.
07-06-2015, 07:56 AM
I'd really like you to explain this, mainly I don't understand why that is so and I'd like to learn. The optic is held on the upper receiver, the bolt and the barrel are too. The lower is your stock and your fire control group. The way I'm seeing it (could be wrong) is even if there is some movement, the barrel and scope are going to reflect this movement as they are part of the upper. The play between the upper and lower (being a few thou off) seems like it would fall within the margin that most high end scopes are parallax free.


Second, what about lapping the face of the receiver? Is that another gimmick? Lapping the bolt too?


And yes correct twist rate is going to be selected, mainly going to shoot .223 hornady 75gr BTHP.

The reason bolt rifles are bedded to attain best accuracy is to eliminate any movement between the action that has the optic attached, and the stock where the shooter attaches. IF you have movement between the upper and lower, what are the chances of the 2 pieces ever being in the same place from shot to shot? Also what are the chances of the shooters eye being in the exact same spot with the optic when the upper moves a couple of thou in assorted directions?
The M14 crowd discovered many years ago that the barrelled action being bedded to the stock and some of the loose parts being welded together makes the rifles shoot more accurately, so I guess my question is why anyone would conclude that the AR platform will defy both physics and logic?
Think of it this way. Add a couple thou movement between the take down pins and the bosses, add another few thou for tolerance between the take down bosses and anchors, add a few more for tolerance around the barrel extension to the upper and all of a sudden you have a large number when the addition is done. Accuracy is reducing the variables, not simply dismissing them as many internet AR gurus tend to.

Pinning on gas blocks even when you have great fixtures is ok for "battle rifle" results but not at all good enough when real accuracy is the goal. Take a look at any of the factory installed FSGB assemblies factory installed from Colt, DPMS, Stag whomever and you will see damned few that are truly perpendicular. Drilling and reaming a tapered hole tends to rotate the FSGB from top dead center, which as seen on many factory rifles where they employ taper pins leads to the front site post not being straight up and down, well guess what? The gas port is also partly covered. And the worst part is there is very little that can be done to correct this problem.
During the manufacture of better quality barrels the gas port is drilled using the 4th axis on a CNC milling machine. At this time it is really easy to rotate the barrel exactly 180 degrees and mill a flat spot precisely opposite of the gas port, this then allows for the perfect alignment of the gas block to the port when proper assembly technique is used. Using good quality set screws allows for plenty of pressure to ensure no gas leakage, assuming that the barrel is machined to tight tolerance for perfect fitment of the gas block. This is not always the case with mass produced goods and becomes more of a problem when guys build Frankenguns under the misguided assumption that all manufacturers use the same dimensions.

Lapping the lugs in on an AR is worthwhile when trying to achieve best accuracy, but is only truly of use when this is coupled with having the chamber cut at the same time to keep headspacing within tightest tolerance.

Lapping a receiver should not ever be required on a decent quality part. I mean really, think about this. IF the part was made properly in the first place 1 should never have to correct dimensional imperfections.
Just like with scope rings, if you have to lap them to get them to fit, they are not very high quality in the first place.

If you were to ask my advice on twist rate for the 75s I would go 8 twist. THAT is exactly what I am running on my own precision AR, which is totally built to shoot the 75 gr Amax bullets and it WORKS!

Hope that helps in your quest for knowledge.