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+--- Thread: Fte, ftf bca 20" grendel. (/showthread.php?tid=23987)
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Fte, ftf bca 20" grendel. - bjmac - 11-28-2021
I just fired my grendel upper for the first time yesterday. 90% of the time it has a failure to feed, 10% fte. When it ejects it is really weak maybe 3 o clock 6". It did this with wolf and hornady black ammo. The lower is borrowed from my 300 bo and I know it's good. The upper is an aero reciever, psa full auto bcg with supplied bca bolt, the barrel is a 20" bca heavy contour, gas block is anderson low pro .750. Last round holds open on bolt body or mag, not sure as I was pissed off but the bolt was beyond the mag follower.
I have to assume it's short stroking due to low gas. Admittedly I did not measure the gas block hole placement to barrel hole as the barrel was dimpled, could the dimple be off for the anderson low pro? I'm in the mountains hunting so I can't take it apart to check until I get home. Just looking for some experience with the block dimple being wrong for the anderson. Or something I over looked. Brass was good after firing, don't believe it's a mag issue, bcg, buffer or spring issue. I'm kinda leaning toward too little gas.
Fte, ftf bca 20" grendel. - VASCAR2 - 11-28-2021
Sounds like to little gas to me. I wouldnt trust the dimples on the bottom of the barrel to align properly with a random gas block. I always measure the distance of the gas port to the shoulder of the barrel. I then verify the gas hole location from the edge of the gas block. I make sure my measurements are correct and set my gas block over the gas port. I use a lead pencil to mark my barrel but always re measure before tightening down the gas block. Fortunately this has always worked for me and I never had to remove a gas block for misalignment.
If you remove the gas block I suggest you measure the gas port. Ive read you can get close using drill bits but I can get a fairly close measurement with calipers. The info I have is a 20 6.5 Grendel should have a .0935 gas port.
Fte, ftf bca 20" grendel. - bjmac - 12-02-2021
So I got home and checked it all out. Everything looks good to me. The only thing I did was drill the gas port to 3/32. It was only a tiny bit smaller maybe a couple thousand. There was an insane amount of carbon for very little shots taken i only shot wolf and hornady black. Weird. Any other ideas?
Fte, ftf bca 20" grendel. - VASCAR2 - 12-02-2021
Unless the gas block was loose youll see carbon leech out but usually seals off pretty quick. The only other thing is if the bolt carrier is out of spec and loosing pressure somewhere. Wolf is definitely dirtier than Hornady ammo in my experience but it doesnt take long for carbon and residue to form under as gas block.
Fte, ftf bca 20" grendel. - bjmac - 12-02-2021
VASCAR2 Wrote:Unless the gas block was loose youll see carbon leech out but usually seals off pretty quick. The only other thing is if the bolt carrier is out of spec and loosing pressure somewhere. Wolf is definitely dirtier than Hornady ammo in my experience but it doesnt take long for carbon and residue to form under as gas block.
Nah, this was in/on the bcg. The gas block is definitely sealed, it still has the oil i put on the barrel under it , aside from where the hole is.
On another note, I was shooting this almost free floating. I mean I didn't shoulder it, it was laying on the float tube resting on a backpack. I had to it was 25 with 30 mph winds and I had a vest on. I was shaking like a leaf. Could it be like a lump wristing a pistol thing? In other words, did the gun movement negate the gas impulse?
Fte, ftf bca 20" grendel. - LR1955 - 12-02-2021
bjmac Wrote:Nah, this was in/on the bcg. The gas block is definitely sealed, it still has the oil i put on the barrel under it , aside from where the hole is.
On another note, I was shooting this almost free floating. I mean I didn't shoulder it, it was laying on the float tube resting on a backpack. I had to it was 25 with 30 mph winds and I had a vest on. I was shaking like a leaf. Could it be like a lump wristing a pistol thing? In other words, did the gun movement negate the gas impulse?
BJ:
The gun movement didn't negate the gas impulse.
That said, it is most likely a gas problem of some sort. Apparently you already have confirmed the gas block is aligned with the gas port and the port is big enough. Check the gas rings on the bolt. If they are in place and not missing chunks of them, the next question is if the gas tube is mating properly with the gas tube on the bolt carrier. Next would be if the gas tube is obstructed or bent. After that you look at the gas key to make sure the gas can get through it and into the bolt. I once had a primer blow and it lodged into the gas key. Impossible to get out and had to replace the gas key. Took hours to figure it out. Sometimes gas keys can loosen up too and gas leaks.
Might as well check all of these things before going on. Not even sure where to go after that other than something not right with the bolt carrier.
Fte, ftf bca 20" grendel. - bjmac - 12-02-2021
Yeah, I already went through it, blew through the gas tube and had flow at the block. Blew through the key with bolt in, no flow. I pulled the bolt blew again and had flow. I like to think I'm pretty good with ar's. The only thing I noticed is the tube doesn't have the normal obvious shoulder at the key end. I checked the fit in the key, it wasn't loose but I've never had the need to check that before so I have no reference and I really don't want to pull any of my others apart to check and mess with accuracy. I didn't see any carbon on the tube near the key like it was leaking back though. I've been dealing with these things for 20 years and this has me stumped. The problem is I can't shoot that often as its an hour drive from home.
Fte, ftf bca 20" grendel. - montana - 12-02-2021
bjmac Wrote:Yeah, I already went through it, blew through the gas tube and had flow at the block. Blew through the key with bolt in, no flow. I pulled the bolt blew again and had flow. I like to think I'm pretty good with ar's. The only thing I noticed is the tube doesn't have the normal obvious shoulder at the key end. I checked the fit in the key, it wasn't loose but I've never had the need to check that before so I have no reference and I really don't want to pull any of my others apart to check and mess with accuracy. I didn't see any carbon on the tube near the key like it was leaking back though. I've been dealing with these things for 20 years and this has me stumped. The problem is I can't shoot that often as its an hour drive from home.
There should be a flare or button on the end where the gas tube engages the gas key. The easiest way to check is with a Mark Browns custom gas tube gauge https://www.brownells.com/gunsmith-tools-supplies/rifle-tools/gas-system-tools/ar-15-m16-gas-tube-gauge-prod41043.aspx
This does not mean your rifle will not work since Olympic Arms never used flared gas tubes with their rifles since they had oversized their barrels gas ports compared to most mill spec. The gas key inside diameter should be .180-.182
The gas tube ends should be .180 -.0006
An easy way to check for gas key obstruction is with a piece of weed wacker line. If it pushes through to the inside of the carrier then you're good to go.
If you do a revers torque test on your gas key screws at 30 inch pounds, you can tell if a screw is broke which can be a major cause of gas loss. Most short stroking is related to gas key problems in my experience, besides a clocked gas block to barrel port..
If you have another bolt carrier that is known to work, it is a simple way of testing if it is a carrier problem..
Fte, ftf bca 20" grendel. - bjmac - 12-02-2021
I checked that tool out... why check the id? I've only ever seen tubes with shoulders on them. Not to mention it doesn't account for different thicknesses of the actual tube. Unless the other end (right side of pic) is for the o.d. of the tube. Just wondering.
Gas key screws are staked so that would be difficult. I doubt 30" lbs would budge the stakes. I could just replace the key but I see no "blow by." I'll check the o.d. of the tube and the i.d. of the key in a few minutes.
Fte, ftf bca 20" grendel. - bjmac - 12-02-2021
The tube o.d. at the step is .1805, the key i.d. is .1825. I tried uploading pics but it says my file size is too large.
Fte, ftf bca 20" grendel. - montana - 12-02-2021
The ID of the gas key is set to mesh with the OD of the flared end of the gas tube. If the gap is out of spec or too big, then you can lose the appropriate gas seal. If the button or flair on the gas tube is worn or has a flat spot, it will be out of spec. It is hard to see if the button has a flat spot next to the receiver ceiling. The gauge will find these. Gas tubes should be within spec .180 - .006. If they are undersized it will create an improper gas seal to the gas block or gas key. If the gas key or gas block holes are also out of spec, it will exacerbate the problem even more.The reverse torque test of 30 inch pounds is purposely set under torque spec of 57- 58 inch pounds. If the screw loosens at this lower setting, then there is a problem. If the screw is broken or under torque spec, then the staking will not matter..
Fte, ftf bca 20" grendel. - montana - 12-02-2021
bjmac Wrote:The tube o.d. at the step is .1805, the key i.d. is .1825. I tried uploading pics but it says my file size is too large.
My pic shows .180 +.0000 -.0006 on both ends..
ID gas key is .180-.181
I use a .182 for a no gauge..
Fte, ftf bca 20" grendel. - bjmac - 12-02-2021
Huh. Let me measure a functional.
Fte, ftf bca 20" grendel. - bjmac - 12-02-2021
I get .1820. On another key (same company)
Fte, ftf bca 20" grendel. - bjmac - 12-02-2021
Just to clarify for everyone that may read this. I'm using digital calipers and the o.d. is pretty hard to get with the flare. I could be off a thousandth.
Fte, ftf bca 20" grendel. - montana - 12-03-2021
I'm guessing you're close enough not to be a problem. The problem with specs is that a person can narrow their focus on them instead of seeing the entire process. I have an older TDP picture from 1970 showing the spec being .1805 to .1813 If a part is over of spec on one end, but under spec on the other those parts will function perfectly.
If your rifle is running that close to the edge to cause a problem, then you have other problems.. It can be a combination of issues that can create short stroking..Over gassed rifles will also have poor ejection and have similar problems. If your rifle is holding open on the last round reliably, you probably don't have an under gassed issue. It is hard to access a problem without knowing all the details..
Fte, ftf bca 20" grendel. - bjmac - 12-03-2021
No, it holds open on the bolt body half the time. Which i thought was weird because i don't get feeding jams. One fte. Only one time did it cycle... the others were empty chambers... rack and fine. It's very odd. It shouldn't be happening which is why I brought up me not shouldering it. The only thing I can wrap my head around.
Fte, ftf bca 20" grendel. - montana - 12-03-2021
bjmac Wrote:No, it holds open on the bolt body half the time. Which i thought was weird because i don't get feeding jams. One fte. Only one time did it cycle... the others were empty chambers... rack and fine. It's very odd. It shouldn't be happening which is why I brought up me not shouldering it. The only thing I can wrap my head around.So it is short stroking. Do you have another bolt carrier that is known to work you can try?
Fte, ftf bca 20" grendel. - bjmac - 12-03-2021
montana Wrote:So it is short stroking. Do you have another bolt carrier that is known to work you can try?
I do that will have to be my next move. Unfortunately it will be months away unless they lax the restrictions at a local indoor range.
Fte, ftf bca 20" grendel. - montana - 12-03-2021
bjmac Wrote:I do that will have to be my next move. Unfortunately it will be months away unless they lax the restrictions at a local indoor range.
I think that is your best move next to see if it is your bolt carrier or gas port size. I forget most people don't have an outdoor range 5 minutes away to function test problem rifles....