Gunsmithing First rifle build.

ZL1 Rider

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Minuteman
Jun 22, 2012
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Hello,
As a new member, I apologize if this is posted in an incorrect location. If so, could a moderator place it in the correct forum section? Thank you in advance.

I am undertaking my first ground up build on a AR-15 based rifle to be used as a test platform and Sniper’s Hide was recommended by some marksmen at the range I belong to as a good place to find answers on specific components for the project.
This rifle will be used to test a SCGS (Selective Cycle Gas System). I have picked the following parts as the main foundation for the build:
1. Krieger barrel 1:7.7 (24”)
2. Mega arms billet upper and lower set.
3. Spike’s Nickel Boron BCG (w/ GP block)
4. Geissele trigger

If anyone can recommend a component replacement that would create a more stable platform, I would appreciate the input.

The bullets tested will be 75gr VDL and 77gr A-MAX.
The targets will be at 200 and 300 yards (these are the range limits).
The desired outcome is for 0.5” or better groups.
If this works, the next step will be a 308 build.

Thank you in advance for any advice that you give.
 
Re: First rifle build.

If you are going to test something, that impies a comparison with something that does not employ the system/concept being tested, for benchmark purposes.

Ideally, the only differing component should be the one being tested. I know this is an added complexity, but it also adds validity to any resulting conclusions.

Greg
 
Re: First rifle build.

I Googled "SFGS" and "selective fire gas system", and the only mention of it on the Internet is your post above. I'm also not sure what you mean by "test platform", or why you would selectively build a .308 dependent on the outcome of building a pretty different rifle.

As far as "stable", you can't go wrong with a Krieger barrel (although 24" is pretty long for an AR shooting to only 300 yards), and the Mega is a good upper/lower set. Bolt assembly is a fine choice, but the trigger will have little to do with a stable platform.

Finally, shooting half an inch groups at 300 yards is about a sixth of a minute of angle, and a pretty ambitious goal for a even a benchrest gun. I'm not sure how you chose this as a goal, but it might be somewhat unrealistic.
 
Re: First rifle build.

Thank you Greg.
Here are some more details on the test setup. The control will be the rifle itself. That construction will remain fixed throughout the tests.
The test comparison is focused on the accuracy differences between the cycle modes of the system.

Mode 1 is a standard gas piston ejection cycle.

Mode 2 is an interrupted ejection cycle. This is to allow the bullet to completely exit the barrel while the bolt remains locked. This is to replicate a bolt action cycle.

The objective is to achieve a noticeable improvement in the accuracy of the rifle (hopefully equal to a bolt action) while maintaining the semi automatic functioning of the rifle. The idea is to reach a bolt action functionality without having to manually cycle the bolt by hand.

Primary testing on "off-the-shelf" ar rifles did decrease the shot grouping diameters. The design principle was tested by comparing the standard gas cycle group to a group that was made with the gas piston disabled. The accuracy improved. But the cost was an inconvenient manual cycling of the bolt with undesirable movement of the shooter.

Mode 2 will allow the shooter to decide when the empty case will be ejected without having to move.

Hope this clears up some of what I am trying to achieve.

I have been told to use a Timney trigger over the Geissele trigger. I have never used a Timney.
Has anyone here used both and can offer an opinion on the pull consistency of each?

It has also been suggested that I use a MIL spec BCG and have the parts REM processed. Any thoughts on this one?

Thanks again for you input.

Eric
 
Re: First rifle build.

jhnmdahl,
Thanks for the input.
The "SFGS" and "selective fire gas system” is just the current acronym that I am using for this design in the lab and reports.

The 5.56 was chosen for stage 1 testing due to cost, familiarity, availability of certain components in the design.

The 24” barrel is planned for a trip to a different range with a 600 meter limit. I currently only have access to a 300 yd distance.

I knew the 0.5” group would be a very tough when I wrote it into the spec. I have learned over the years that if I don’t set difficult goals then I can get lazy and not push myself to reach an objective as best as I can.

Also, the 0.5” goal is not one of the projects critical goals. If the design makes it then it will be icing on the cake. If it doesn’t then it will be a learning experience and create an opportunity for improvement in the future.

Thanks again,
Eric
 
Re: First rifle build.

If you're looking to test the pure accuracy of the rifle, why not knock the target back to 100yards where wind and other environmental factors have an effect and observe MV differences to gauge any affects caused by a varying dwell time of the round? There's a good likelihood you'll run into a wall at about 1/2 moa due to the inherent accuracy of the other components of the rifle. I would get it built up by a very good AR smith who will lap the upper receiver and do all the other things he can to squeeze precision out of the rifle and install your gas system, then test and send it back to him to get the other gas system installed. You can do these things yourself, but every little thing is going to count if you want that kind of precision, and he'll knock out any deviance a less experienced smith or armorer might induce.
 
Re: First rifle build.

While the acronym is unfamiliar, the concept has been discussed here a long time back. I suspect that this series of tests is going to follow a similar outcome to your initial series.

One of the points that was critical back when was the lag time between gas port reveal, and perceptible action movement. Basically, the bullet was gone before the BCG began any motion. I've looked for any references, but it was apparently too far back.

I believe that high speed cinematography could be conclusive.

You know, if you really wanted to be shrewd about this one, you <span style="font-style: italic">could</span> pose the question to <span style="font-style: italic">Mythbusters</span>...

Greg
 
Re: First rifle build.

I've used both triggers and prefer the Geissele High Speed National Match DMR because it's fully adjustable and a 2 stage. As far as the bcg, I would get the bolt that was used to headspace the barrel (matched set). Then you'd have to figure out if you want the m16 or ar15 carrier. As far as any special coatings on the carrier, I'd just get one that's hard chromed only because it's so easy to clean. If this was a hard use rifle, the boron coating would be more desirable. I'd also look to get either RRA, Palmetto or Spikes lpk. Only because these are the only ones I know of right off hand that use spiral pins instead of split pins (personal preference I guess). I think you have a decent build sheet. Have fun.