And now for something completely different...

[stagpanther]

Now that I'm satisfied with my grendel project--I'm returning to this one--which has proven to be a bit of a nightmare--I'm learning that all the components have to be "just right" or you're in for a merry-go-round of jammed bullets and failed bolt lock-backs.

bfoosh wisely suggested I look into buffer adjustment possibilities--and I'm certainly going to look into that--these things are absolute murder on the bolt catch levers.

One lesson I've already learned--if I had to do it again there is no way I would commit to a lower that restricts you to glock magazines only. I would go with lower magwell design that is optimized for reliable feeding of the cartridge and then select the best magazine set-up. At the very least--I would select a lower that has the flexibility to adjust the types of magazines. The glock 20 magazines I've used had to be heavily altered in order for me to get them to work--there's too much play in the mag well as is and that will introduce an endless cycle of adjusting.

One thing I finally figured out (it takes me a while) is that the bolt does not actually strip and feed the cartridge in the same way a typical lugged AR bolt does--what it really does is "release" the cartridge from the follower and feed lips--and then the cartridge simply pops up into the upper--hopefully to be directed by the bolt into the chamber. In my set-up--because of the alignment of the glock mag--the cartridge pops up well in front of the bolt--and this I believe contributes poor alignment of the cartridge on the way into the chamber--often jamming the cartridge at angles in the front portion of the chamber. In my latest modification I've trimmed the magazine feed lips back so that the timing of the "pop" puts the cartridge closer to the front of the bolt--hopefully reducing the likelihood of jamming the cartridge.

[stagpanther]

switched out the 3x scope for a 3 x 9 buckmaster--my favorite cheapo special scope when nothing else is available.

Took the rifle out just to zero the scope @100 yds--but I might change that to 50 later. I also did some work on modifying the glock magazine as well as repositioning the extractor to allow the mag further height inside the upper.

I had a few spare nosler hollow point loads as well as a few mild 115 gr lehigh loads just for zeroing--because it was snowing so hard I decided not to bring the labradar and test the whole ladder of loads I had made--it has trouble enough tracking pistol cartridges out of a rifle with a compensator that vents the muzzle blast straight forward.

No issues with the hollow points--but the xtreme pentrators have some difficulty chambering--though it was just the first one I dropped the bolt on that didn't feed right--the next two cycled fine and the bolt locked back when the mag was empty. Although 3 shots don't make a group--and one of them flew pretty wide--still, two of them drilled in almost exactly at the POA--so I'm at least more encouraged than I was before. The cartridges are still taking damage on the way into the chamber--the spent cases show signs of that, and I still have a ways to go to reduce that--but at least I'm starting to get closer.



Another lesson learned--shooting at a white target stuck in a snow bank in heavy snow isn't the best high-visibility idea.

[stagpanther]

Went out again today with high expectations--and the bolt jammed on the very first cartridge I dropped it on. Not happy to say the least, packed up and went home and called joebob's to see if anyone might have any further suggestions. The guy I talked to was very courteous and assured me that they would do whatever it would take to make me happy--and I believe he was being genuine. After discussing the issue(s) for a while, he let drop that the 9mm's and 45 acp's have something on the order of 1/10th the problem calls that the 10mm has--not surprising judging what I've been through so far.

So here are a few of the things I've learned that I think make a difference in tuning a blow-back AR--at least the 10mm.

One of the key tuning items is the positioning of the ejector hook--it does lots of things--helps position the magazine, works with the bolt catch, provides a rail for the bolt to ride on and ejects the spent case. What I finally learned to do was position it as high as possible without impeding the bolt's movement, by lightly loosening the retaining screws, closing the bolt, and then gently pushing a magazine up against it--this should get an ideal height of the magazine while still allowing the bolt to do it's thing.

Even after this I was still jamming cartridges--not to mention the two or three bolt catches I've broken so far. The toll in damaged cases and bullets was getting quite high--especially with the lehigh bullets which are quite expensive.

After polishing the chamber mouth (per KVP's recommendation) I reduced the issues a little bit, but was still getting intermittant jams--and even the successful chamberings were showing signs of damage to the case and bullet.

Then I noticed something which is so obvious and simple and yet as far as I know hasn't been mentioned on any forum or manufacturer's website!! (I think I should win an award of some sort lol).

The magwell cut into the lower is symmetric for accommodating the glock magazine--but if you look very, very very closely at the magazine when inserted in the magwell--there is actually a slight off-set to the side in the position of the cartridge at the top of the magazine relative to the chamber.This makes a big difference since there are no feed ramps to engage the cartridge. The glock magazine tapers towards the top--which probably doesn't help.

So I took a couple of pieces of fuzzy side velcro tape and positioned them inside the magwell so that they leveraged the magazine into a straight presentation relative to the chamber mouth and the very first cartridge I dropped the bolt on--a difficult to chamber lehigh penetrator--chambered perfectly and when extracted showed virtually no signs of damage!! Eureka! (I hope)

Another thing I would recommend--forget about dropping the bolt on the first round feed from a magazine in a blow-back AR--the bolt, buffer and spring are so robust it's not needed to chamber a round--especially since there are no bolt lugs to lock up. I would simply hand charge it back enough to clear the back of the magazine from a closed bolt sorta like you charge an AK.

Will test soon--hope I'm right.

[Stats Shooter]

This is why I considered making a 45 or 9 mm. But alas, I am setup to load 40s&w by the bucket full...And really love the cartridge. So, that's what I am building. Plus, once I'm done, I will cured of any desire to ever build another.

[stagpanther]

This is why I considered making a 45 or 9 mm. But alas, I am setup to load 40s&w by the bucket full...And really love the cartridge. So, that's what I am building. Plus, once I'm done, I will cured of any desire to ever build another.

My bolt actually has 40 S & W stamped on the side. The biggest plus I see--is the ease of barrel swap; lacking a gas system--j

[stagpanther]

When the cartridge "stutters" upon being cycled--in other words it drags on the chamber wall due to a bad feed angle, I've learned you can generally tell by feel. It's my belief that this extra drag could potentially allow the cartridge to be fired slightly out of battery. Considering there is no bolt lock up--and the mass of the bolt itself is considerable when you think about it in terms of it traveling back at your face--I decided to add a little piece of plumber's tape to the ejection port edge as a sort of "in battery indicator" in case the feed felt odd.

[stagpanther]

Bought a brand new unmodified mag and tried it--it still intermittently jammed with the lehigh loads, so I think that means you still have to modify the mag a bit as well (if your lower jams cartridges like mine)--I dremmelled the feed lips back so that the cartridge "pops" closer to front of the bolt--I also dremmeled down the contact surfaces of where the ejector and bolt catch hook engage the mag--allowing the mag a bit of extra height inside the upper.

[stagpanther]

Finally got out to test my extreme penetrator loads yesterday--labradar says these are running right at 2000 fps in the warmer loads. It also says that the velocity, and hence kinetic energy, falls off pretty rapidly.



The first two magazines functioned perfectly--and then the cartridges started jamming again--alas my eureka buzz was dashed along with my hopes of finally figuring out the secret to happiness with this set-up.

My final analysis, after spending $750 in parts for the build and another $200 in replacement bolt catches and wasted reload components--is that this is a fundamentally flawed design concept for the 10mm cartridge--my conclusion is that there is simply too much play between the magwell cut in the lower and the slender, tapering glock magazines which present the cartridge at a high angle to the chamber. Getting the gun to run reliably becomes an endless cycle of adjustments.

My advice--stay away!

[Stats Shooter]

I wonder how these
"Just right carbines" work then in 10mm. I can't imagine they would sell it with as many issues as you are having.

The 40s&w and 9mm seems to work great from everything I have read. Perhaps it's because they are both shorter. The 45 acp versions work well too from what I have seen.

[stagpanther]

I wonder how these "Just right carbines" work then in 10mm. I can't imagine they would sell it with as many issues as you are having. The 40s&w and 9mm seems to work great from everything I have read. Perhaps it's because they are both shorter. The 45 acp versions work well too from what I have seen. __________________ Formerly known as as screen name "Mississippi"

I'm sure some custom guys--and higher end outfits have figured out how to tune these just right--but if you've actually worked on a 10mm blow-back AR that uses a glock mag--you would know what a precarious balancing act it is.

[stagpanther]

Not wanting to admit defeat and relegate this rifle to over-priced paperweight duty--I tried a "hail Mary pass" and removed the last round bolt hold back pin altogether from the lower and fired off a test mag of 165 gr sierra bullets. Other than the slight inconvenience of losing the hold back ability--everything seems to work fine (I know, how many times have I said that, right), most of all the cartridges look and feel like they are getting cycled with little to no damage, and presumably without the bolt lock-back the bolt catch destruction should halt. Small prices to pay to keep the gun running.

[stagpanther]

Felt confident enough to whip up a batch of 165 gr "flying ashtrays." I hope they chamber and fire OK.

[davidsog]

Nice looking gun, Stagpanther!

After discussing the issue(s) for a while, he let drop that the 9mm's and 45 acp's have something on the order of 1/10th the problem calls that the 10mm has--not surprising judging what I've been through so far.

Not surprising. Chamber pressure and engineering complexity have a direct relationship in blowback designs!

[stagpanther]

Thanks--it's actually a very simple set-up--my first stab at a blow-back build, and I'm very partial to 10mm since I've had a glock 20 for quite some time (though it spends much time in the guise of a 9 x 25 dillon now). Speaking of PTR's the ejector hook which the bolt rides on reminds me a heck of a lot of the one on a PTR functionally. The downfall of this particular design, IMO, is the pin/rod which tries to do too many things--engages the magazine, holds the bolt catch etc. and if it's slightly out of tune the whole cycling system can easily be thrown out of whack. If I did one of these again (and probably won't, unless maybe a 9 x 25 dillon) I would get a lower that didn't enslave you to glock magazines only.

[stagpanther]

Went out today and fired off all the ladder charges without a single jam--removing the hold back pin did the trick--so that's the good news.

the cases are showing slight deformation at the case mouth--but I'm not feeling any "ker-klunk" as the cartridges chamber so it may be they are getting a bit of damage on the way out. It was about zero degrees with a wind chill well below zero--so I'm not sure the numbers are really representative of what the cartridges can really do (165 gr deep curl driven by accurate 9) but I was getting velocities in the range of 1650 to 1750 fps--which for a pistol bullet isn't that bad. Labradar caught most of them--enrgy figures are nothing spectacular--as a real game dropper (like deer) I wouldn't be very confident with it past 75 yds--and probably would stay at 50 yds or in. Someone on another forum said something like "what's all this fuss over the 10mm--my 7.62 x 39 is much better" and that is certainly true. Not being DI--this kind of set-up gets pretty kicky in recoil as you get up into hotter and larger bullets too. I guess it's biggest selling point is you can fight it and your glock with the same magazine.

meh. big deal lol

[Big Shrek]

I'm still wondering, for what you planned to pay for it all and the headaches that ensued...
why not simply buy the JustRightCarbine in 10mm for $736?? Seems like that would have
kept you from having all those fitment issues...

https://www.cheaperthandirt.com/prod...ategory=515588

A buddy of mine went with the Glock 20 Mechtech kit, he loves the heck out of it.
Simple, easy, no headaches...

[stagpanther]

I'm still wondering, for what you planned to pay for it all and the headaches that ensued...
why not simply buy the JustRightCarbine in 10mm for $736?? Seems like that would have
kept you from having all those fitment issues...

Cause I'm stupid?

[Big Shrek]

Originally Posted by stagpanther

Cause I'm stupid?

Some of us are gluttons for punishment...
probably why I married a redheaded retired Marine gal

And why my aunt would always call me, "You Big Dummy!"...

[stagpanther]

It was a learning experience--always wanted a 10mm carbine--and now that I actually have it--that itch has been thoroughly scratched.

[davidsog]

Thanks--it's actually a very simple set-up--my first stab at a blow-back build, and I'm very partial to 10mm since I've had a glock 20 for quite some time (though it spends much time in the guise of a 9 x 25 dillon now). Speaking of PTR's the ejector hook which the bolt rides on reminds me a heck of a lot of the one on a PTR functionally. The downfall of this particular design, IMO, is the pin/rod which tries to do too many things--engages the magazine, holds the bolt catch etc. and if it's slightly out of tune the whole cycling system can easily be thrown out of whack. If I did one of these again (and probably won't, unless maybe a 9 x 25 dillon) I would get a lower that didn't enslave you to glock magazines only.

Not only are they asking a lot out that pin:

The more powerful the cartridge the faster the bolt velocity/mass of the bolt = the more forces on that pin and harder it is to get it to work with stoppages/malfunctions.

It has a quite a bit in common with the delayed roller blowback, too.

Nice gun!

Some of us are gluttons for punishment...
probably why I married a redheaded retired Marine gal

And why my aunt would always call me, "You Big Dummy!"...

Like my first ex-wife....

Pain is a powerful teacher....

[Big Shrek]

Pain is just weakness leaving the body! HUAHH!

Dang Drill Sgt's...jokers had an answer for everything

[stagpanther]

The more powerful the cartridge the faster the bolt velocity/mass of the bolt = the more forces on that pin and harder it is to get it to work with stoppages/malfunctions.

The real issue IMO is balancing the bolt mass/ buffer weight and spring--versus reliable cycling and keeping the cartridge reliably in battery--lacking any kind of bolt-to-chamber lock-up mechanism and no gas system to dampen the blow-back. In that respect--the heavy stuff is a necessity. But even the simple lock-back is MURDER on bolt catches--I got one of the hardest steel ones I could find short of titanium--and even it got deformed a bit, and even if it resisted; that energy is going to go someplace--which includes deformation of the bolt-catch slot cut into the aluminum upper--that is if the lever itself doesn't snap first right about where it is pinned to the receiver.

This set-up is about 75% of the way to a real world pistol carbine IMO. For all it's difficulties, the blow-back system has one great advantage over just about any other carbine design with a gas system--and that's a true quick screw on/off barrel swap out. I could see a bit of refinement to bolt and buffer system modular flexibility should make multiple pistol caliber swaps pretty easy. Lastly, I would "get over" the fixation on "fight the magazine" (meaning don't make the design dependent on just glock magazines)--rather make the optimal ergonomics in the lower configuration and magazine arrangement for the best, most reliable presentation of the cartridge to the chamber. The present designs have the cartridges simply too far away from the chamber, at odd angles, that without any kind of ramps the probability of the cartridge taking some damage, hence affecting the accuracy--goes up. Just my opinions, of course.

[stagpanther]

The extreme pentrator bullets continue to show really good accuracy out of the carbine, usually in the 1.5 to 2 MOA range--that's pretty good for a pistol bullet out to 100 yds in my book.



Unfortunately, the lehigh bullet also shows the greatest propensity for jamming in the process of cycling from the magazine--I'm still getting a rate of about 1 in 5 cartridges hanging up. The conventional jacketed hollow points do not do that--but I'm sure they are still taking some abuse on the way in--and therefore probably affecting the accuracy.