First AR build questions

[mattg521]

Getting close on my first AR build. A couple of things don't seem to be working out right.

First: The bolt hold feature does not seem to work. Not when fired as I haven't gotten that far yet. When I manually pull the operating lever with an empty mag inserted BCG still flies forward into battery. This happens even if I "help" the bolt hold with my thumb. BCG spec issue? Could this indicate a buffer spring issue?

Second: Free float handguard is contacting gas block/front sight base. It appears that it is even preventing the block from riding all the way home to its stop on the barrel. It seems to me this invites an undergassed situation and probably will be worse for accuracy then a standard handguard. I plan to go after the parts of the handguard with a dremel and file to relieve the areas of contact and insure full seating of gas block unless I here different advice here.

Some info on the build:
Precision Aero upper and lower
RRA lower parts kit
R guns BCG
AIM Industries Free float mid length hand guard
Troy and Magpul mags

Any advise and or abuse will be welcome.

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[iamme]

1) Get some pics of your lower so we can see what's going on with the catch. It's simply a pivot that blocks the bolt and shouldn't be causing problems and may be a "user error" situation.

2) Look at a "standard" upper and you'll notice in behind the gas block there's a cap. Going free float you don't use this cap but you still need to account for that gap in the barrel/shoulder. What gasblock/gas system is the gun? Again pics would help as well.

 

[NoAim]

Let's address one thing at a time:

Bold Hold Open. The bolt hold open is just a little block that pops up and rests against the bolt face. So my first instinct is to ask whether if the bolt carrier is going back all the way. The BHO should hold against the bolt face, and not the bolt carrier.

This can be a common issue if the buffer tube length and buffer size aren't matched (i.e. using a carbine buffer tube and a rifle buffer).

 

[mattg521]

Let's address one thing at a time:

Bold Hold Open. The bolt hold open is just a little block that pops up and rests against the bolt face. So my first instinct is to ask whether if the bolt carrier is going back all the way. The BHO should hold against the bolt face, and not the bolt carrier.

This can be a common issue if the buffer tube length and buffer size aren't matched (i.e. using a carbine buffer tube and a rifle buffer).

Thanks. This is a very helpful approach.
BCG goes all the way to the rear in that bolt face clears the bolt hold by Approx. 6mm (1/4"). I saw a post on another forum suggesting that disconnector spring could be switched with bolt hold spring. I bought the lower "complete" so I can't speak to assembler talent. This would presumably cause bolt hold to barely engage the lowest edge of locking lugs on bolt face. This appears to be exactly the case when I look at my firearm. I am planning a disassembly at least to the point of determining correct spring placement. Any additional input is very welcome.

 

[mattg521]

2) Look at a "standard" upper and you'll notice in behind the gas block there's a cap. Going free float you don't use this cap but you still need to account for that gap in the barrel/shoulder. What gasblock/gas system is the gun? Again pics would help as well.[/QUOTE]
Ah, I hadn't thought to consider the width of the hand guard cap in the original assembly. Perhaps my concerns about undergass due to hole misalignment was overblown. Still accuracy "improvement" due to free float would seem obviously lost due to this impingement. Thoughts?

 

[coop44]

help: chamfer the edge of the gas port hole with a conical dremel bit. this will stop any concerns about alignment.

Abuse: this thread is proof some folks shouldn't be allowed access to ANY tools.

 

[iamme]

Still accuracy "improvement" due to free float would seem obviously lost due to this impingement. Thoughts?

Ummm, what's the question? You lost me there!

 

[mattg521]

Ummm, what's the question? You lost me there!

Sorry. I was referring to the hand guard touching the gas block/FSB and assuming that it will negatively affect accuracy.

 

[iamme]

If you have a freefloat it won't- unless you're running an extended rail on an A2 FSB.

This is from the WOA install page but will help explain what I'm talking about. WOA does their shoulders with FF tubes in mind so they'll but up.
Screw the float tube onto the barrel nut. You want to screw the tube far enough onto the nut that when the front sight is installed you have clearance between the end of the tube and the front sight base. Keep in mind that if you have a GI contour barrel, the front sig ht will be about .035 away from the shoulder when you install the pins due to the fact that we no longer have the handguard cap sandwiched between the shoulder and front sight. Keep this in mind when you slide the front sight on to check your clearance. If you have a WOP or WOA barrel, they were contoured with a float tube in mind and you will want to slide the front sight all the way against the shoulder.

 

[DirectDrive]

It's a little tricky to get the bolt catch roll pin in or out.
I use a special shaved roll pin punch made just for that one pin.
You don't want to volunteer to monkey with that one if you don't have to.

Someone used to handling/building ARs could probably diagnose and fix your two issues in short order.
Why not call someone like Curt's Guns in Milwaukie and see if he can look at it ?
Just make sure that whomever you contact is AR competent.

 

[mattg521]

Update:
Removed Bolt catch and found what appeared to be the correct spring. Careful study of the bolt catch shows the lower only allows the bolt catch to go up a lot further than when the upper is installed. Is there supposed to be a feature machined into the inside of the upper to allow more upward rotation of the catch? If not perhaps my bolt catch is out of spec.?

Removed via dremel enough material to create an actual separation between the hand guard and FSB. Had to remove a bunch of Al to actually get clarence. Also accommodated the .035 offset suggested earlier in this string. When I measure the fore end cap in my possession my caliper says it is only .025". Any input about that? Unless I hear that I need to move the FSB in another .010" I am feeling pretty comfortable about the hand guard/undergass issue.

 

[DirectDrive]

With your calipers measure the distance from the barrel's shoulder to the barrel's gas port.
Now measure the distance from the rear edge of you gas block to its gas port.
Subtract the smaller number from the larger.
That is your gap between the GB and the shoulder.

Also, while its open measure your barrel's gas port diameter for reference.

 

[mattg521]

Update:
Finally got out to the woods today. Apparently my removal of hand guard material worked. Gas system works great. Gun feeds and cycles. Only thing it won't do is catch the bolt. Thanks for all the wisdom and advice so far.

 

[iamme]

What's ejection like? What buffer, what length gas system? Pics of the lower with upper off showing bolt catch? Does the catch move freely? A pic with an empty mag inserted, upper off would help as well.

 

[mattg521]

Sorry no pix ability. Catch moves freely without upper installed. One shooting session has left tiny marks on the inside of the upper and on the catch. Only question in my mind is which of the two parts is out of spec. I'm hoping for the catch obviously since it is cheap and easy to install. Is there supposed to be a feature machined into the inside of the upper so that the catch can achieve full swing?

 

[DirectDrive]

Sorry no pix ability. Catch moves freely without upper installed. One shooting session has left tiny marks on the inside of the upper and on the catch. Only question in my mind is which of the two parts is out of spec. I'm hoping for the catch obviously since it is cheap and easy to install. Is there supposed to be a feature machined into the inside of the upper so that the catch can achieve full swing?

Try using Image Shack or Photobucket.
You create a free account at one of these sites, then upload a pic to your "My Images" file on that site.
Choose the image, then copy the Forum Code and paste it into your thread here.
I use Image Shack and resize my pics with Irfanview (free) before uploading.
Image files are huge straight out of the camera, so they need to be trimmed down before you can do any forum posting with them.

There is no relief cut on a forged upper for the bolt catch paddle.

The first thing to check, with the rifle assembled is.....

1) Pull back the charging handle.
Can the bolt be locked back manually over an empty mag ?
If yes, then the bolt catch and magazine are probably OK as long as the catch is holding the bolt and not the carrier.


2) At the range, with good, full power 5.56 ammo, put one round in a mag and charge the weapon.
Fire the rifle with one round in the chamber over an empty mag.
Does the bolt hold open after that round is fired ?
If it doesn't, you (in most cases) have an under-gassing issue or some other problem with your rifle's action.
Could be as simple as a misaligned gas block or a buffer that is too heavy.

 

[mattg521]

Thanks DD. Will not catch bolt even with manual assist. Catch barely engages lowest edge of bolt face. It almost holds with strong pressure applied to catch but doesn't quite hold. Fails when fired with either.223 or 5.56mm. When cycled manually the bolt face clears the catch by about 1/4" and still fails to hold. I am going to try a new catch.

 

[DirectDrive]

Thanks DD. Will not catch bolt even with manual assist. Catch barely engages lowest edge of bolt face. It almost holds with strong pressure applied to catch but doesn't quite hold. Fails when fired with either.223 or 5.56mm. When cycled manually the bolt face clears the catch by about 1/4" and still fails to hold. I am going to try a new catch.

Don't commit to driving in the second set up until you confirm....

> spring/plunger bore is free of debris
> correct spring
> plunger in correct position
> function check using shank of correct diameter drill bit for temp pivot
> roll pin correct diameter and length
( I recently found a bolt catch with a bore that was too small !)

I tape the drill bit in place and let the roll pin chase it out.
This keeps everything aligned during the roll pin driving process.

 

[iamme]

If the catch isn't catching either way don't sound like a gas issue. I'm going to guess someone used the wrong spring/plunger on the catch preventing full movement, or just a faulty catch.

 

[mattg521]

Thanks for all the advice guys. Got a replacement catch today (thanks Skydiver) and the issue has been resolved. Geometry of the two parts was slightly but clearly different. Glad to have successfully completed my first build. Now if I could just find some ammo I'd be a happy camper.