Problems with McGowen barrels

[Shivesy]

I have 2 Grendel barrels made by McGowen Barrels and I am having issues with them both. Actually I purchased 3 of them but 1 of the 3 seems to be okay. The issue I am having with 1 particular barrel is that I am pulling the bullet away from the unspent cartride when I use the charging handle to unload my rifle. The other barrel can be tough when removing the round but not to the point of pulling bullets from the case. Now this is a little tricky and I will try to explain myself better. I can hand cycle a 10 round mag for both barrels for example and all goes well and everything is fine. Now when I hunt and shoot lets say 5 of the 10 rounds and put the rifle on safe, then drop my magazine with 4 rounds left, here's where the problem starts. I have one more round left in the chamber. This round can be difficult to extract. I have actually pulled the bullet from the case a few times. Sometimes is worse than others. I am using factory grade Hornady A-max or SST 123 gr. ammo. No reloads at all.
I have sent the 2 barrels back to McGowen along with a 10 round mag, 10 SST rounds and a bolt approx. 2 weeks ago and today finally taked with their gunsmith who manufactured these barrels. He says that he used a 6.5 Grendel PTG reamer and the barrels are within specs and he could not duplicate my problem. He seems to be lost at what to do and so do I. He suggested that I have a ammo issue. I disagreed but anyway McGowen suggested that I send more ammo so they can do further test. Any suggestions??

[Bike Effects]

After you retrieve the bullet, is it marked?

[Bill Alexander]

Unless the tooling is measured and inspected against the correct drawing with tolerances included you can only say that you think you have a correct chamber. The alternative is to employ Replicast compound to examine the chamber directly. Cerrosafe while widely used is not the best choice as it does move slightly. Measurement needs to be with an optical tooling comparator. Calipers or even a micrometer will not let you resolve angles or determine any barreling that might be present.

[Shivesy]

Bike: yes the bullets are marked.
Bill: thank you for you response. I doubt that anyone at McGowen will go through the trouble to do what your suggesting. I can't understand why I can do a manual cycle and the rounds extract wihtout a problem, but then when I fire off a few rounds I can't extract the unspent round that is left in the chamber. When I am able to remove the round via charging handle it is very difficult and I sometimes have to use the mortaring technique.

[Bwild97]

Shivesy, You may want to give this a try!

From Stokesrj,
The Carbon deposited in the throat is baked on at higher temperatures so as the barrel is cleaned, the solvents soften and remove the carbon from the bore at a faster rate than they do at the throat. The result is a slow build up of hard carbon, easily visible with a bore scope but very difficult to detect without one. This ring adds resistance to the bullet at the critical time of initial acceleration. This can result in rapid increase in pressure, a spike. It can also result in bullets vaporizing before reaching the target. I've seen it more often at 600 yards but have also seen it at 300. I had it happen to me at 300 yards during my rapid prone string at the Nationals one year resulting in a 9 yes, which means they were all in the nine and ten ring but one of the ten didn't make it there. My scorer said he saw a blue puff before the bullet got to the target.

I took my upper in to vendors row where they have a lot of Hawkeye bore scopes and had them take a look. Sure enough a carbon ring had formed at my throat. This was on a Krieger barrel with about 3,900 rounds on it. I JB'd it and inspected again, no carbon ring and none since. I now JB the throat every 200-400 rounds to keep it in top condition.

Here is how Frank White of Compass Lake Engineering says to use JB to maintain the throat.

1. Work the JB into the patch.

2. Wrap the patch around a worn bronze bore brush (not nylon).

3. Using a bore guide, stroke back and forth for 5 strokes in the first half of the barrel. (Breech Half) Push patch through the muzzle and unscrew brush from rod and carefully remove rod from barrel.

4. Remove old patch from brush.

5. Repeat steps 1 - 5 for 5 complete cycles.

Note: A neglected barrel may require this to be repeated up for many more cycles until it is smooth.

Note: The patch will never come out clean as JB is a slightly abrasive compound and is working to polish your throat and bore.




This cured my tight throat issues.

[Shivesy]

Bwild...that's all good info but should that need to be done on a new barrel?? I don't think I should have to go to that hassel. Thanks for sharing though..

[jawbone]

very helpful post, bwild. many thanks.

[Shivesy]

Do these reamers come out of spec sometimes? I'm just thinking the Grendel reamer is not right. Maybe the gunsmith isn't reaming deep enough?

[Shivesy]

Bingo!!!! Just hung up the phone with Dave at PTG. (Pacific Tool and Gauge) They made the reamer for the Grendel that McGowen used on our barrels. Dave stated that the Grendel reamers are made to SAMMI specs but the specs drawings are wrong in his opinion. To tight of tolerance. Dave also stated that he has heard of many many case of bullets being torn from the case when this reamer is used. When the barrel throat gets just the slightest of carbon fouling in it the bullet sticks. As soon as I said Grendel reamer Dave new exactly what I was talking about. The Grendel is a double throated reamer and he said to use a .264 LBC reamer and that reamer works just fine and much better with the Hornady Grendel ammo because this reamer uses a parallel throat. *Dave also stated that one could use a uni throat reamer and that reamer would open the throat a bit and probably solve the problem I am having.
This whole subject is way over my head and I am trying to gather info in order to solve the problem of rounds sticking on these Grendel barrels I have purchased. I hope I am on target with this post and if not please correct me.

[Overwatcher]

Pretty clear your bullets are jamming into the throat/lands and something is out of spec.
Slit 2 sides of a new/sized piece of brass with a dremel,stick a bullet in and push the round in with a dowel until it stops.
Then with a coated rod from the muzzle..lightly tap the case/bullet out and measure you OAL.
This will determine your bullet to lands distance.
There is a thread here on how to make a case for this..works slick.
Typically you want .015-.020 jump,anything less and trouble can happen when the throat gets dirty,especially with an auto loader.Also the Grendel's double throat loves to grab and hang on to bullets too.
If the OAL isway long then you can prove to mcgowen that the chamber is out.

[Nukes]

IIRC, the CZ Custom Shop uses McGowen barrels for their 6.5 Grendel conversion of the CZ 527 Carbine.

[Bill Alexander]

The SAAMI spec reamer has plenty of clearance, it was designed to be extremely tolerant of bullet design, to the extent that many accuse it of being a machine gun chamber. The tolerance for the reamer is not specified within SAAMI only the minimum chamber size. The tolerance is the toolmakers descision.

The chamber is set to allow the guns to run excessively dirty and the carbon fouling deposits have clearance.

Hornady ammunition and the 123 Amax was designed for the Grendel chamber. Hornady is the SAAMI sponsor for the Grendel.

You have just found that you do not have a (correct) Grendel SAAMI chamber.

[Keep The Change]

Taking Bill's explanation of the SAAMI spec reamer having plenty of clearance and that the tolerance is the toolmakers decision, along with "Dave's" explanation of the tool having too tight a tolerance, it seems to me that the chamber is too tight. Is this because the tooling is worn and not cutting/reaming as much material as it should or is the tolerance on the tool at or below the small side.

Throw in a little heat and carbon and things get sticky.

Send it to Bill and have his guys run their reamer through it.

[Bill Alexander]

For HSS reamers, which are usually best and cut smoothest when the number of barrels are lower, I would always recommend Manson Precision Reamers. They make fine Grendel reamers and on some occasions even have them as a stock reamer. Do opt for the live pilot though if chambering a good barrel as the fixed pilot can mark the lands as it turns.

I would think that the chamber is too tight especially if PTG say they have no margin on the tolerance. The tooling is essentially out of specification just as soon as it has settled in. This is a good practice for bench rest gun but not for an AR. The Grendel is frequently misunderstood and those chasing accuracy will tighten head space and also the neck diameter and neck length. The cartridge is intrinsically accurate and while the effects may be realized in a specialized bolt action this is not the case with the cartridge in the AR. The gas port abnormality in the rifling has a greater effect. We also see the chamber concentricity is important but that the crown is surprisingly tolerant.

[Bill Alexander]

Perhaps the thread title is a little strong. This is not so much a problem with a McGowen as a series of observations. Upon consideration the chamber is a little tight but this is commensurate with anyone building a fine barrel. A little examination yields the idiosyncrasies of the chamber in hand.