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AR-15 tech help!

2K views 25 replies 15 participants last post by  Johnboat 
#1 ·
Hey guys, I bought a new Olympic Arms AR-15 with the 20 inch bull barrel. Just past the hand rail is a rail mount that encircles the barrel. The rail (for mounting something) is only on one side and facing up. The gas tube runs into this and there are two ports for it to dissipate. Question is: I want to mount a bipod to the rail but the rail is not facing downward. I have been told from a few gun people that I can just rotate the rail around to face downward to mount a bipod. However, this gets rid of the gas tube... Is the gas tube all that important to have??-- afterall, it does have the two ports. Is it needed to aid in ejecting the shell?? I need to go to the range and see but I don't have time for another 2 wks... Any help would be appreciated. I might be able to post pics later if I can find my camera. Sorry if I explained this poorly. Thanks in advance for advise or if someone has done the same thing...
 
#3 ·
You may need to replace the fore stock with a quad rail or something, if your bipod attaches to some type of picatinny rail. Or....a smithy that knows AR's may be able to replace the gas block with one that will suit your needs. I have yet to see an issue with AR's that can't be solved.
 
#6 ·
The block is called a gas block. It is made with rail slots to mount a Back up iron sight or BUIS. you may not move it or the gun will stop working.
Does your rifle have a quad rail, that is, a forearm with the rails on 4 sides? if so, then a a GG&G bipod. if not reply here and i will tell you the next step.
Jim
 
#12 ·
Brio -

Brownell's, Midway, Cheaper Than Dirt are a couple of websites to check out. The part you are referring to is called the gas block. You will need a 2-rail or a 4-rail block to mount a bipod. A reputable gunsmith can swap this out for you while you wait, provided they aren't swamped. The two ports you are seeing aren't ports - it's a roll pin that holds the gas tube in place in the gas block. No gas out of those holes.

http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=188739

Gas tubes are sized to fit specific lengths of barrels: carbine, mid-length, rifle length, etc.
20" is rifle length - Brownell's has them.

BTW - I wouldn't take any more advise from anyone who told you it was ok to rotate that block.
 
#17 ·
I think you have good advice from Ernest, I have a similar gun with a bipod. It's mounted to the free float fore end with a swivel. DPMS or any gun emporium should have one. A quick drill, tap, screw in, do not forget the red loctite and it is GTG.

Don't cheese out and buy a cheap bipod either. We shoot a lot, with folks who shot a lot. # 1 accessory tossed aside, as a POS, are cheap bipods. Harris makes a nice one it's about 130 bucks though. When the bargain 29.95 model falls off your buddies gun, you can be happy you spent the extra.

MM
 
#18 ·
Thanks for the bipod advice marshman. It's amazing how much you learn when something potentially goes wrong. I ordered a quad rail gas block and went ahead and got another gas tube with roll pin. Should be here later this wk.

Might need some advice on installing the new gas tube into the gas block if it isn't as simple as it appears. I am all ears.

Thanks again everyone on being so helpful so quickly
 
#19 ·
I'm telling you, the photo looks like free floated handguards. If thats the Oly 20 inch with a stainless bull barrel with the knurled aluminum hand guards, its more likely than not free floated.

You are going to kill, simply kill, any advantage of the free float the hand guards by mounting a pod on the gas block. I don't know how to say it in any stronger language. POI will vary based upon whether you are using the bi-pod or not, and the POI will vary based upon the downward force applied onto the pod. You will go from having a very accurate weapon in the AR-15 platform, to an extremely heavy spray and pray device at any distance.

Again, buy a swivel stud kit, install it, and then install your bi-pod. Harris, more specifically, to the swivel stud. No need to mess with the gas tube, no reason to mess with the gas block.

If you want to mount stuff off the fore end in addition to a bi-pod, like a light or whatnot, just buy and install hand guards with rails. Now, I think those hand guards with all the cheap junk hanging off them are for mall ninjas, posers, and dedicated zombie hunters, but nonetheless, its still a better approach to use rails on the hand guards if messing with a free floated front end.
 
#23 ·
In response to your inquiry, when shooting off a bi pod, the weight of the front half of the gun will be supported by the pod. Hold the back half up with either your off hand or a bag. So, there is an upward pressure on either the barrel (if mounted to the gas block) or the fore end. Its like putting the sand bags under your barrel instead of under your hand guards. With a free floated fore end, that pressure is transmitted back to the front of the upper receiver instead of applied to your barrel.

Rule number one basically is to avoid any pressures applied - up, down, side ways, whatever - to the barrel if at all possible. Hanging 3.5 pounds off the barrel three quarters of the way down the tube is rarely, if ever, good for accuracy. Same reason you never want the sand bags to touch your barrel.

Plus, it changes the harmonics of the barrel.

Pressure on the barrel 3/4 of the way down the tube will cause the POI to change relative to off hand shooting with free floated handguards or shooting off sand bags that are properly positioned under the hand guards. The weight is now off the barrel and the front end is supported at the leading edge of the upper receiver.

Further, when shooting off a bipod, you typically lean into the bi-pod and "tension" the legs. This gives you a solid shooting platform and keeps the scope cross hairs from bouncing (as much). By leaning into the bi pod, you are now applying yet another irregular and inconsistent force to the barrel if the bi-pod is mounted to the gas block. Different amount of lean, different point of impact downrange. Don't lean one time, different point of impact.

Even a tight sling wrap on a non free floated hand guard can change the POI.

Oly uses good barrels, and with decent handloads, this could be a .75 MOA weapon or better. Way different than these close quarters weapons where changing POI don't mean nothin cause the range is across the room or 40 yards.

Do a quick google on free floating, and all this and much, much more will be explained in exacting details.
 
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