Small problem with free float handguard with rail gas block.

[CASEYPM]

I am building a PSA PA-10. I'm nearing completion and just lacking barrel and bcg. I've bought what was described as a 9" dpms free float tubular handguard and in actuality it's 9.5". I have a gas block with picatinny rails on it and I tried fitting it the upper with handguard on with gas tube in and found that the gas tube doesn't go into the upper far enough. It's about 1/2-5/8 too short into the upper. So my question is: what is the standard length of a mid-length free float handguard that will allow an A2 fs or gas block with rails? And does the 3/16" of barrel nut the handguard screw into count as handguard length? I just need to know how much of the handguard I need to shave off so it'll have a flush fit. I don't want a low pro gas block because I want to absolute cowitness my red dot and the handguard doesn't have rails. Thank you in advance for your help and advice.

[CASEYPM]

I know I could just cut off 5/8" of the handguard and I'd probably be good, just wondering if anyone know the exact length the handguard should be.

[CASEYPM]

[blue109]

The gas block will only work in one spot. Doesn't matter how long or short the tube is. You need to install the barrel with the gas block on it, and use that as your template to trim the tube. Some gas blocks may sit a little deeper than others, and the barrel nut won't be in the same spot with the barrel in place vs just threaded onto the upper.

[CASEYPM]

Gotcha. That's what I was thinking. I was wanting it ready to go when my barrel and bcg come in. I'll wait until the barrel is on.

[survivalshop]

  Are you sure you don't have a Carbine Gas tube , you will need a Mid Gas tube to work with a Mid gas system . The Mid length Hand Guard length is 9.5 ". What Gas system is the Barrel you are getting ? 

Edited August 15, 2015 by survivalshop

[guruofhotrod]

When I built one of my uppers, (DPMS High pattern) I installed a Samson Star 10 in a 9.2" length:

 

http://www.samson-mfg.com/mm5/merchant.mvc?Screen=PROD&Product_Code=STAR10-9-2&Category_Code=SHOP

 

Along with the optional DPMS Barrel nut:

 

http://www.samson-mfg.com/mm5/merchant.mvc?Screen=PROD&Store_Code=SM&Product_Code=DPMS-BN&Category_Code=308AR_STAR_ARMALITE

 

Along with a PRI Railed gas block - even though the parts were designed for a Armalite AR10 they were a close enough fit 

that with a little "massaging" they worked out well for the mid length DPMS type platform I had to open up the anti-rotation

tabs on the bottom of the rail where it locates against the front of the upper receiver and with a counter bore I relieved the

back of the gas block to tighten up the gap between the gas block and the front of the rail (always check the overlap of 

the gas key and the end of the gas tube to ensure the tube isn't bottoming out in the gas key when in battery) (to check

I slide a small O-Ring on the end of the gas tube inside of the upper and then install the BCG, lock it into battery, remove

it and measure the distance from the O-Ring to the end of the tube and compare that to how deep the socket in the gas

key is by inserting a loose gas tube into the gas key until it bottoms out, marking the location on the gas tube at the edge

of the gas key then measuring from the mark you made to the end of the tube.) With this combo I used a standard mid 

length gas tube.

Edited August 15, 2015 by guruofhotrod

[CASEYPM]

@survivalshop it is a mid length gas tube. I am getting a mid length 16" barrel. I think I'll wait until it comes in the mail to do anything, but just from fitting it all together, it looks like it won't work. Very frustrating because I checked all the specs before purchase and it's not lining up. Is AR-10 gas port lengths different than AR-15's? I had reason to believe they were the same.I included a pic of the tube next to a ruler and from the back of the gas block to the end, it's 10.75".

Edited August 15, 2015 by CASEYPM

[CASEYPM]

@guruofhotrod thanks for the tips and links. I will definitely do that.

[guruofhotrod]

Looks like your combo is going to work out fine - from the front face of the upper (at the rail) to the rear face of the gas block 

you should be around 9-5/16" - as blue109 said install the barrel, gas tube and block first to determine the space you have

for your handguard/rail then trim the rail ONLY IF NECESSARY - "measure twice cut once" absolutely applies here, your mid 

length gas tube should be around 11-3/4" in length (end to end) Also, when using the O-ring method to measure the gas key

to gas tube overlap, make sure you remove the O-Ring before you try to fire the rifle (you don't want it breaking loose and

possibly jamming things up) 

Edited August 15, 2015 by guruofhotrod

[CASEYPM]

Here's a better representation of the problem.

[guruofhotrod]

Looks like your handguard may wind up a little long but, it's difficult to know for sure without getting

a barrel mounted and checking the location of the gas block relative to the end of the handguard.

When I measured my upper with a BHW 16" mid gassed barrel installed I came up with the 9-5/16" 

measurement from the front of the upper receiver (where the gas tube enters) to the step on the

barrel that locates the gas block. (this dimension can be affected by the manufacturing tolerances

of several components though) 

Edited August 15, 2015 by guruofhotrod

[CASEYPM]

The gas port on the gas block is exactly 5/16 from the back end of it. I guess I'll see when the barrel gets here. The gun shop I go to is inside a machine shop. I'm pretty sure they're more qualified to make the cut on the handguard than me with a hack saw.

[survivalshop]

    You need the Barrel to make a final fitting to be sure . Worse case , assemble & torque hand guard Barrel nut & if you need to make clearance for the Gas block rail on your Hand Guard , your in a machine shop , make clearance . The gas block should seat at the barrels gas block flange .

  I think it should be fine , but as has been said , there are no MilSpec specifications for 308AR's & spec's could go here or there from Manufacturer to Manufacturer 

 

 When completely assembled, your gas tube should protrude into the Upper receiver to the middle of the Cam pin lock in the Upper receiver.

Edited August 16, 2015 by survivalshop

[CASEYPM]

It all worked out.... I hope. I ended up only cutting 1/4" off the tubular handguard and not 5/8" I'm glad I got that measurement right. Then after assembling it, it looked as though there was another problem. The gas tube was still a bit short inside the upper. I searched all over the web and found a post on ar15.com stating that was normal for that in the PA-10. The gas port on the barrel was exactly 9" and tube was 11-3/4". Take a look and tell me what you think. Palmetto state says that their gas tubes aren't proprietary and are the same as carbine, mid and rifle from the AR-15. 

[98Z5V]

I think I'd pick up the longer Armalite gas tube, and give it a fitment try, if it was mine.

[CASEYPM]

Isn't theirs 12.25? That pic is at an angle to avoid the flash shining back. It's only 1/4" short. 

[survivalshop]

That position looks too short , the end of the Gas tube protrusion should come up to the middle of the Cam Pin channel in the Upper receiver . You may get short cycling problems if it protrudes less . It has to do with timing , if the Carrier Key separates too early from the Gas Tube , gas pressure may not be enough to complexly cycle the Action , because of gas pressure drop . 

 You may have to test to see if this happens , but we have seen this before , where there has been a problem.

 

 

 

Edited August 28, 2015 by survivalshop

[CASEYPM]

Well, I'm ordering a BCG today and in the meantime, I'll head up to a local gun shop that specializes in black rifles. Hopefully they'll have an armalite AR-10 gas tube to try out. 

[CASEYPM]

I think I'd pick up the longer Armalite gas tube, and give it a fitment try, if it was mine.

you're absolutely right. I just took the time to measure it and an armalite 12-1/6" tube will fit perfectly. I just ordered one. Thanks. 

[98Z5V]

This is just more proof that PSA doesn't know what they're doing, in the 308AR game...

[CASEYPM]

My PA-10 build just got pushed over the $750 mark with this purchase. So if it will run like a sewing machine and punch paper at 100 yds, I'll call it winning. I was thinking about it last night how much research I've put into it to not have problems, and so far this is the first. Well besides the DPMS handguard being too long. Their staff is ignorant and I'm sure they only follow a troubleshooting algorithm on their computer with no real world experience with their products. No regrets so far. 

[CASEYPM]

The dot on the ruler represents where the armalite gas tube will sit, which is 5/16 longer than what psa recommended. Can everyone agree that this is where it should be? I talked to a friend with a PA-10 which he bought built from PSA and has the same short gas tube and he's having problems short cycling on all ammo. 

[98Z5V]

 I talked to a friend with a PA-10 which he bought built from PSA and has the same short gas tube and he's having problems short cycling on all ammo.

SS called that one - exactly right.

I think you'll be fine with the Armalite gas tube. 

[CASEYPM]

Thanks to you guys, I'm becoming a bit of  an armchair expert at a gun I've never even shot. I'd be pretty screwed if it weren't for joining 308ar.com.