AR Build help

Boondock Saint · Mar 25, 2010

Now that I played with my lower (upper is still in the shop), I can't move the selector back to safe unless the hammer is cocked back. Seems like it wouldn't be an issue, since the bolt would presumably re-cock the hammer after every shot. :shrug:

Scott's Gunsmithing · Mar 25, 2010

Boondock Saint said:
Now that I played with my lower (upper is still in the shop), I can't move the selector back to safe unless the hammer is cocked back. Seems like it wouldn't be an issue, since the bolt would presumably re-cock the hammer after every shot.

Its normal, no worries.

BTW if you want a full polish job to compliment your upper, we need your lower too, so I can contour and polish the hammer.

I should have it done by tomorrow evening.

Boondock Saint · Mar 25, 2010

Scott's Gunsmithing said:
Its normal, no worries.

BTW if you want a full polish job to compliment your upper, we need your lower too, so I can contour and polish the hammer.

I should have it done by tomorrow evening.

Good deal. Thanks for the info, and of course the great work! I can't wait to see it.

HoCoShooter · Mar 26, 2010

Calengor said:
You should know that they're currently out of your item 4 b, the miad/ctr/dd lpk one.

yep, I found out when ordering. I couldn't find the daniel defense in stock anywhere and ended up getting a Stag kit with a leftie selector switch :thumbsup:

and got the miad, ctr and some buis from magpul direct.

HoCoShooter · Apr 29, 2010

Look what UPS finally delivered today. The lower is on it's way to Dan by Monday and I should be done in a couple weeks! I'm pretty excited

300RUM · Apr 29, 2010

Congratulations, looks great :thumbsup:

Smokey45 · Jun 18, 2010

I'm slowly building my own AR for competitions, 3-gun I hope, I have an Eotech XPS2 sight, Magpul CTR stock, Rock River 2-stage NM trigger group, midwest 1-piece handguard, and MBUIS sights. I have two questions for everyone here:
-I have a heavy barrel in 1x9, will a 1x7 barrel make a significant difference?
-Does anyone have a CMMG gas piston kit? How do you like it?

Calengor · Jun 18, 2010

Smokey45 said:
I'm slowly building my own AR for competitions, 3-gun I hope, I have an Eotech XPS2 sight, Magpul CTR stock, Rock River 2-stage NM trigger group, midwest 1-piece handguard, and MBUIS sights. I have two questions for everyone here:
-I have a heavy barrel in 1x9, will a 1x7 barrel make a significant difference?
-Does anyone have a CMMG gas piston kit? How do you like it?

I believe 1:7 will stabilize a wider range of bullet weights, from around 50-80, whereas 1:9 will only properly stabilize the lighter ones.

I think consensus on pistons is as follows: go with an LWRCi or prepare to yourself for problems.

canuck · Jun 18, 2010

Calengor said:
I believe 1:7 will stabilize a wider range of bullet weights, from around 50-80, whereas 1:9 will only properly stabilize the lighter ones.

I think consensus on pistons is as follows: go with an LWRCi or prepare to yourself for problems.

1:8 may serve you best. As to the piston question: LWRCi is great for sure. If you want a retrofit kit, buy the adams arms unit. I have one and have run it hard at 2 Pat rogers courses with absolutely no issues.

JamesBailey · Jun 18, 2010

Smokey45 said:
-I have a heavy barrel in 1x9, will a 1x7 barrel make a significant difference?

Using Greenhill's Formula to solve for best twist rates, I calculate that a 9:1 is best twist for a 0.224 bullet w/ length of ~0.84", which in weight is roughly 64grains.

A 77gr SMK w/ length of 0.995" is best served w/ a twist of 7.5:1 according to Greenhill.

Here is the formula:

Twist = 150 * D^2 / BL
where D = bullet diameter in inches
and BL = bullet length in inches.

canuck · Jun 18, 2010

JamesBailey said:
Using Greenhill's Formula to solve for best twist rates, I calculate that a 9:1 is best twist for a 0.224 bullet w/ length of ~0.84", which in weight is roughly 64grains.

A 77gr SMK w/ length of 0.995" is best served w/ a twist of 7.5:1 according to Greenhill.

Here is the formula:

Twist = 150 * D^2 / BL
where D = bullet diameter in inches
and BL = bullet length in inches.

correct. which is why I like the 1:8.

JamesBailey · Jun 18, 2010

canuck said:
correct. which is why I like the 1:8.

However, at the distances Smokey will likely be shooting at - Eotech distances, 100-300m - a 9:1 will shoot a 77gr fine. Only at longer ranges >300m will the under spin have any impact on the observed accuracy.

I wouldn't worry about the barrel twist Smokey, especially if its a heavy barrel (as you cite), and otherwise a well manufactured piece.

Get the gun built and get out there! :thumbsup:

bulletbill · Jun 18, 2010

The problem with Greenhills formula is that it fails to take into account the sape of today's bullets nor velocities of modern ammunition. When it comes to modern ballistics Millers formula is, well, at least current.

An excel spread sheet with the formula already done so that the numbers are plug and play is here; http://accurateshooter.net/Blog/millerformula.xls

Using Millers formula it suggests that driving a 77gr SMK at 2800fps (which is acheivable) and 1:9 twist provides a stability factor of 1.44 which by the formula is enough to stabilize the bullet.

JavaDan · Jun 18, 2010

HoCoShooter said:
cool thanks, I was more concerned about the quality of the lpk kits. I wasn't sure who actually manufactured either but heard a lot of lpk's are made by continental I think it was.

I figured I couldn't go wrong with either magpul.

Another really good lpk option is the one sold by Spike's Tactical.

http://www.spikestactical.com/z/index.php?main_page=product_info&cPath=104_97&products_id=426

They also have an "enhanced" lpk should you want it.

Spike's assembles their lpks from parts chosen from distributors who, in their opinion, have the best quality parts, e.g. trigger, springs, pins, etc..

I originally bought a DPMS lpk but decided to get the Spikes before my assembly. I'm glad I did - it was much better quality than the DPMS.

Russ D · Jun 18, 2010

Smokey45 said:
I'm slowly building my own AR for competitions, 3-gun I hope, I have an Eotech XPS2 sight, Magpul CTR stock, Rock River 2-stage NM trigger group, midwest 1-piece handguard, and MBUIS sights. I have two questions for everyone here:
-I have a heavy barrel in 1x9, will a 1x7 barrel make a significant difference?
-Does anyone have a CMMG gas piston kit? How do you like it?

For 3-gun I'm not sure you want to be shooting heavier bullets with more recoil anyway.

Why do you need or want a gas piston?

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