View Full Version : AR Barrel Bore Pin Guage
What pin guage(s) is/are best for confirming an AR .223 barrel isn't under-bore?
I'm trying to isolate the reason a barrel is shooting high velocity and popping primers. I measured jump to lands, and that matches a known good (Colt) barrel. So I'm down to tight bore, or non-5.56 chamber.
Pin guages are cheap, so I'd like to eliminate the tight bore possibility before spending the money on a 5.56 Chamber reamer.
SHOULD at .223 pin guage pass through an AR 5.56 barrel??
I just ordered 4 pin gauges from .221 to .224. That should cover it well no?
the pin gauge isn't going to be able to tell if the chamber is cut correctly...
Ned makes a gage that can tell you if it's 5.56 or not. Much cheaper than the reamer too.
the pin gauge isn't going to be able to tell if the chamber is cut correctly...
I know that. What I'm trying to avoid is buying an expensive tool that I don't need. So if I pin the bore and that checks out, I know the last possibility is the chamber. I'm 99% sure it's the chamber, but with my luck, I'd buy the reamer and it wouldn't fix my issue.
Ned makes a gage that can tell you if it's 5.56 or not. Much cheaper than the reamer too.
I would buy the gauge if I ran across a lot of problem barrels, but I just have the one.
There is also bore area, that is part of the SAAMI print. It needs the bore and land diameters, as well as the land and grove ratio to calculate.
It is a better way to measure the size of the hole the bullet goes through for pressure concerns than the land diameter, but it’s not something that is easy to measure.
the AR-15 Junkie
03-01-24, 06:10
I just ordered 4 pin gauges from .221 to .224. That should cover it well no?
None of those will fit, to large.
joedirt199
03-01-24, 07:00
There is that metal stuff that you can melt into the chamber and bore to measure what is going on. Can't remember what it is called but it is low temp soft then hardens when cooled. I have an ingot of it but never used it. It is for making chamber casts.
Been there Mark.....
I understand your trying to save money, but Neds' 5.56 reamer is more then worth the scratch! I've found 6920 barrels that were short (probably end of reamer life).
The only way I know of easily (a relative term) finding bore diameter is slugging the barrel. I've done this and it was a real pain; and didn't really help much.
YMMV
Thanks, all! Yeah.. I've read about chamber casting and slugging. Both were too much ass ache for me as a non-gun smith. My reluctance is due to the barrel being contracted for the Mil. BUT... perhaps the barrels were rejected for this reason. Who knows the real back story on "over-runs".
Looks like I'm just going to have to get the reamer.
Ned Christiansen
03-01-24, 08:40
A .219 or .218 pin ought to fit. My bore erosion gage is .220 so it is basically detecting any erosion of the bore diameter to .220 or greater.
If you have a bore scope, and with all the things you do I think you oughta, you can use it to detect a short freebore / throat.
Thanks, Ned. So a .220 pin passing through would be a problematic situation? (nothing to do with my situation, but still interesting)
With simple pin gauges, you can only measure the bore diameter of .2190+.0010
This 5.55mm precision ground rod works nicely after you chamfer the ends.
It has a .2185-.0002 diameter, which gives a nominal clearance of .0005"
It is almost 4" long, so it doubles as a straightness gauge too that checks a better straightness than the mil spec straightness gauge at 6" x .2175", which seems a little sloppy.
https://www.mcmaster.com/catalog/130/4278/2900A323
I just ordered 4 pin gauges from .221 to .224. That should cover it well no?
ViniVidivici
03-01-24, 11:29
There is that metal stuff that you can melt into the chamber and bore to measure what is going on. Can't remember what it is called but it is low temp soft then hardens when cooled. I have an ingot of it but never used it. It is for making chamber casts.
Cerrosafe.
joedirt199
03-02-24, 09:38
Yup that's it. Thanks Vini.
Got the McMaster pin Clint posted above, and WOW! This barrel has a tight spot about the first inch of rifling in front of the chamber. Explains everything. I mean the gun cycles like a perfectly gassed 14.5, but still pops primers. The initial spike/overpressure has dropped by the time the gas system is actuated.
This pin is perfect for gauging bores. It passes perfectly through a Colt barrel and fits like a glove.
titsonritz
03-04-24, 20:31
Got the McMaster pin Clint posted above, and WOW! This barrel has a tight spot about the first inch of rifling in front of the chamber. Explains everything. I mean the gun cycles like a perfectly gassed 14.5, but still pops primers. The initial spike/overpressure has dropped by the time the gas system is actuated.
This pin is perfect for gauging bores. It passes perfectly through a Colt barrel and fits like a glove.
I’m going to add one to next order from them.
Just pinned all my AR barrels. Only the Colts pass the gauge all the way through with no resistance. A Sabre 14.5 middy will pass it with some force.
Most of the barrels stick about 1" into the rifling. That's curious. I'm wondering if that part of the rifling gets roughed up with wear. Both Colts are relatively new barrels.
Great, now I'm going to have to pin gauge all my rifles.
Thanks MarkM.........
Just pinned all my AR barrels. Only the Colts pass the gauge all the way through with no resistance. . . Both Colts are relatively new barrels.
dumb remark here, but are the barrels clean? That can make a big difference ime
Great, now I'm going to have to pin gauge all my rifles.
Thanks MarkM.........
I may not have the experience to read this situation correctly. It's surprising that so many barrels stick right at that first inch of rifling. And I don't mean a little resistance. The gauge has to be reversed by tapping it with the cleaning rod to get it loose.
dumb remark here, but are the barrels clean? That can make a big difference ime
Both Colts were dirty from shooting on Sunday. The Minnies was thoroughly cleaned since I was possibly going to run the reamer in that chamber.
bobcatdriver
03-05-24, 08:08
I may not have the experience to read this situation correctly. It's surprising that so many barrels stick right at that first inch of rifling. And I don't mean a little resistance. The gauge has to be reversed by tapping it with the cleaning rod to get it loose.
I didn't read this right the first time. The gauge will not pass through at all without force? You do have to reverse it? That would be disconcerting.
I didn't read this right the first time. The gauge will not pass through at all without force? You do have to reverse it? That would be disconcerting.
Correct. My psycho side wants to pound that fukker through. Hard to see from the upper receiver, but that all appear to have about a 1/2 inch of rifling visible where the gauge stops. I'm pushing the gauge from the muzzle back towards the chamber.
Just pinned all my AR barrels. Only the Colts pass the gauge all the way through with no resistance. A Sabre 14.5 middy will pass it with some force.
Most of the barrels stick about 1" into the rifling. That's curious. I'm wondering if that part of the rifling gets roughed up with wear. Both Colts are relatively new barrels.
Copper and Carbon accumulation....I don't see what else it could be!??
It seems a tighter bore would show signs faster along with a shorter lead??
What do you think Mark?
Copper and Carbon accumulation....I don't see what else it could be!??
It seems a tighter bore would show signs faster along with a shorter lead??
What do you think Mark?
Not sure. I'm no barrel expert. I really don't want to go down the bore scope worm hole. I was guessing maybe some fire cracking/roughing of the first inch of rifling raises some material on the lands?
At this point, I'm still thinking of adding the reamer to remove all chamber concerns in all my non-Colt barrels. If only the Minnies barrel failed the pin gauge, I'd consider the case closed, but now I'm still puzzled.
Many barrels are not as straight as most people would think.
I expect that you may be measuring copper buildup, and maybe some less than straight barrels.
Many barrels are not as straight as most people would think.
I expect that you may be measuring copper buildup, and maybe some less than straight barrels.
I'm going to try a copper removal on the Minnies tonight! Thanks!!
I remember seeing FN's machine that straightens barrels that failed their pin gauge. Scary.
triggerjerk
03-05-24, 13:50
I remember from somewhere that .22 lr's are reputed to develope a carbon ring just in front of the chamber you are supposed to deliberately attack every now and then. Don't know about centerfire....
Carbon ring in centerfire is in the leade area I believe. This gauge wouldn't catch that. But I did try cleaning the hell out of the barrel.
bobcatdriver
03-05-24, 16:24
Looking back at the original MMC thread, it seems several forum members bought the barrels. It would be nice if more would chime in if they've had any issues.
I purchased one of Ned's reamers to use on four barrels and then sold it for almost what I had in it.
I re-cleaned this barrel with some bronze brushing, and also some Copper remover. (Bore Tech) The patches didn't turn very green at all. A little copper fouling, but nothing like a bolt gun.
I used some magnified glasses and light and can kinda see that the lands are just roughed up from normal (i guess) firing.
I'm back to the Reamer at this point. I think that my higher use barrels are the same, AND.. at some point the colts will even get worn to where the gauge sticks at the starting lands.
Looking back at the original MMC thread, it seems several forum members bought the barrels. It would be nice if more would chime in if they've had any issues. I’ve got one. It seems fine. M855, XM855, some rando stuff. I’ll try to remember to bring it out and shoot something stupid through it soon.
Mark, have you heard of “Final Finish” ammo? I think Tubbs or Zedecker sells it on a website. Its an abrasive impregnated projectile claimed to “fire hone” or something like that. I have no idea if it’ll work, or destroy your gun or what; just curious if people try this as a remedy. There’s also some sort of abrasive paste. We could be deep into bad idea territory though.
I’d personally start with Ned’s gauge, then reamer if it fails, then back to head-scratching or doing something drastic like lighter ammo.
I thought of those things.. Steel jacketed ammo, Even buying PMC Bronze to run in it. If it were the only barrel that the pin stuck in at the first inch of the chamber, I'd explore those deeper. But almost all of my older barrels stick.
Thinking bore scope and/or reamer is the next step.
bobcatdriver
03-14-24, 17:30
I got a couple of the rods from McMaster a week or so ago, didn't test until yesterday because I was waiting to smooth the sharp edges. I mean, what the heck, it's a cheap tool. The rod will drop right through all 4 barrels of my current shooters. They are the MMC, Spikes 16", Colt 16" and Brownells 11.5". The Spikes has the highest round count, then the MMC, then Colt, then Brownells. I'll have to check my son's then my son-in-law's.
I mean, what the heck, it's a cheap tool. The rod will drop right through all 4 barrels of my current shooters. They are the MMC, Spikes 16", Colt 16" and Brownells 11.5". The Spikes has the highest round count, then the MMC, then Colt, then Brownells. I'll have to check my son's then my son-in-law's.
No kidding? The 5.55mm pin right? Almost NONE of mine pass through... as noted. I have not chamfered the pin completely. I just mildly spun the sharp edges with some 400 grit.
Wrap a couple layers of tape around the middle of the pin.
Chuck it in your cordless drill and spend a few minutes with the sharpening stone and oil.
Use the side or back of the stone and keep it moving around.
Flip it around and repeat on the other end.
No kidding? The 5.55mm pin right? Almost NONE of mine pass through... as noted. I have not chamfered the pin completely. I just mildly spun the sharp edges with some 400 grit.
bobcatdriver
03-15-24, 09:19
Wrap a couple layers of tape around the middle of the pin.
Chuck it in your cordless drill and spend a few minutes with the sharpening stone and oil.
Use the side or back of the stone and keep it moving around.
Flip it around and repeat on the other end.
I put them in my drill press, let it run on a slow speed. Then I used a dremel type grinder and a round disc grinding stone, put it on the slowest speed, and used very light pressure on the edge with the flat part of the stone. It took several minutes, and I got nice smooth 45* angle. Not wide, just enough to see.
Thanks, guys. I may work on that on Sunday. Curious if that will make a difference in pass thru. Although as it is, the Colts still pass the pin completely. So there's some real differences between the barrels.
I'm really interested to see if at some point the lands on the 6920 get roughed up to where the pin won't pass. So I man only chamfer one end of the pin for now.
Just got a wild hair and gauged both the .223 bolt guns we run, and they BOTH easily pass the pin. The Rem 700 5R has possibly 15-20k rounds through it, and the Tikka has maybe 500 rounds.
Still stumped at why so many of the Chrome lined barrels choke at the first inch of rifling.
5.56mm (0.219") is the bore or land diameter. 0.223" (5.66mm) is the groove diameter. The latter affects accuracy more than the former.
Groove diameter cannot be measured with pin gauge. One needs to slug the bore for that.
-TL
Sent from my SM-N960U using Tapatalk
I love this pin gauge. My problem is I don't know how badly the diameter is. Just that the gauge won't pass through. I can somewhat tell by the velocity spikes which barrels are the worst however.
https://i.imgur.com/CbzUmX6.jpg
0.223" (5.66mm) is the groove diameter.
Read more, post less.
https://www.ar15.com/media/mediaFiles/28568/SAAMI_223_chamber_drawing_001-3166432.jpg
...
Read more, post less.
https://www.ar15.com/media/mediaFiles/28568/SAAMI_223_chamber_drawing_001-3166432.jpg
...Off by 0.001" (40um). Stand corrected. Not to argue about the tolerance. Thank you for the warm welcome. :)
Point is measuring bore with a pin gauge serves limited purpose. May get more useful information slugging the bore instead. Cheaper too.
-TL
Sent from my SM-N960U using Tapatalk
Point is measuring bore with a pin gauge serves limited purpose.
limited, but massively critical for me. Tight spots are exceedingly problematic.
limited, but massively critical for me. Tight spots are exceedingly problematic.Slugging can also identify whether / where there are tight spots, either in the bore or in the grooves. Knowing is perhaps the end of the road. Nothing really you can do about it other than replacing the barrel.
-TL
Sent from my SM-N960U using Tapatalk
I've never slugged a barrel so I can't comment on the value of it. But I just need to trouble shoot a few barrels that pop primers on ammo that isn't hot enough to be popping primers.
This $5 gauge gets me some good, basic idea of which barrels that have tight spots and takes 30 seconds to check.
With the pin I don't get the complete story, but I know if I'm barking up the right tree for practically no time or money.
joedirt199
03-23-24, 08:29
I have slugged pellet barrels before and it lets you know if you have a tight spot and also how tight the choke is, if the barrel has one. You can also measure the pellet afterwards and get your diameter since pellets come in several sizes. You would need to find some slugs that would be larger than the barrel. Most .22 slugs are .216-.218. Silicon spray and punch that slug through the barrel.
The tolerance is in the plus dimension. . .
The tolerance is in the plus dimension. . .True. But OP has tight spots in his barrels. The point I was trying to make is groove diameter is bigger. A pin gauge can't get to it, either .223" or .224".
Anyway, I have slugged barrels with lead balls from shot shells or even lead sinker. Didn't think of pellet. A .22 pellet can be squashed fatter to fit.
-TL
Sent from my SM-N960U using Tapatalk
My assumption is that the land and groove relationship is parallel even in the tight spot. In other words, the groove is the same depth, but there's a narrow spot in both land and groove at the tight spots. But I could be wrong.
My assumption is that the land and groove relationship is parallel even in the tight spot. In other words, the groove is the same depth, but there's a narrow spot in both land and groove at the tight spots. But I could be wrong.Hard to say, depending on how the barrel is made. I can only say the depth is not necessarily constant. Slugging will tell you both diameters. And of course everything is hunky dory if you just read the specs.
Traditionally riflings are cut. The bore is predrilled, and cutter is pulled through the bore to cut the grooves in multiple passes. Now there are many different ways to manufacture. But for handloading, with cast bullets especially, and gunsmithing, slugging and chamber cast are the standard methods.
-TL
Sent from my SM-N960U using Tapatalk
Not really a good assumption, especially if it was single point cut, broached or button rifled.
Generally, the pilot for these methods is at or just under the minimum bore diameter, so when it passes through the bore the grooves get cut or cold formed fairly uniformly, but the reamed bore keeps its irregularities.
bobcatdriver
03-24-24, 11:05
My assumption is that the land and groove relationship is parallel even in the tight spot. In other words, the groove is the same depth, but there's a narrow spot in both land and groove at the tight spots. But I could be wrong.
Did you bore scope them? Sorry if I missed it. Just curious if there are any anomalies that can be seen. Not that they would necessarily cause a tight spot.
No. We don't have a scope.
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