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Old May 14, 2017, 05:10 PM   #1
stagpanther
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6mm predator underway

Very excited about this (and I'm a bit jaded when it comes to AR builds ) as most of the parts and components have come in, though the barrel and bolt still haven't arrived. Not sweating that since if this ARP barrel is anywhere near as good as the 6.8 I bought recently--this should be a stellar build and well worth the wait.

The only slight drawback is that there is no factory ammo or reloading dies available for the 6mm predator (yet--hornady should have dies out in the fall), but the ARP website suggests using a grendel bushing die with a .268 bushing and replacement 6mm expander. My first go with a bushing die--and I'm happy to report that it worked perfectly--resizing looks as good as a custom die and neck tension on the bullets was perfect and consistent through every cartridge.

Coming up with theoretically appropriate loads seems easy enough, I just took the grendel data in quickload and used it as a starting point and adjusted everything down for the 6mm bullets and saved as a new entry for 6mm predator. Since these are my "first break-in" cartridges, I didn't even push the loads to the "wild side" but QL projects I should be getting in range of 2800 fps and 1700 ftlbs of energy at the muzzle. That is very respectable for a charge of under 30 grs. in an AR 15 platform.

These are 95 gr ballistic silvertips driven by 8208 XBR.

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Old May 14, 2017, 05:49 PM   #2
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Quote:
Originally Posted by stagpanther
QL projects I should be getting in range of 2800 fps and 1700 ftlbs of energy at the muzzle.
You should be able to do better than that, The 6mm Mongoose can get 2800fps from a 95gr with less than 26gr of Benchmark in a 20" barrel, and that's a smaller case.
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Old May 14, 2017, 06:39 PM   #3
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I'm sure I can push them faster--these loads are well under the max rated pressure--and since they are newly resized I just want to fire-form the cases for the time being and use them for barrel break-in. Once I get a better feel for everything I'll start pushing things.

Quote:
You should be able to do better than that, The 6mm Mongoose can get 2800fps from a 95gr with less than 26gr of Benchmark in a 20" barrel, and that's a smaller case.
I tried tracking down those kinds of numbers--couldn't find em--can you point me to them? Seems like the mongoose is bigger--not smaller, from what little I've been able to find.
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Old May 15, 2017, 08:12 PM   #4
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Looking forward to the thread.....

Especially after seeing this on the 6.8 Forum:

http://68forums.com/forums/showthrea...r-range-report

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Old May 15, 2017, 11:44 PM   #5
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Originally Posted by stagpanther
I tried tracking down those kinds of numbers--couldn't find em--can you point me to them? Seems like the mongoose is bigger--not smaller, from what little I've been able to find.
The Mongoose is definitely smaller. It's based on the 223, opened to 6mm, shortened to 1.69, with a 40 degree shoulder, and blown out to a TCS taper. Here's some data:

https://docs.google.com/spreadsheets...?usp=drive_web
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Old May 16, 2017, 12:07 AM   #6
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.136 bolt recess?? The barrel specs state .135. I decided it was worth the extra money to get a matched and headspaced 750 extreme duty bolt and extension from ARP. That's some nice shooting though, a 55 gr bullet must be a small flat base cooking along somewhere in the 3000 fps velocity range I would think.
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Old May 16, 2017, 12:53 AM   #7
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Quote:
The Mongoose is definitely smaller. It's based on the 223, opened to 6mm, shortened to 1.69, with a 40 degree shoulder, and blown out to a TCS taper. Here's some data:

https://docs.google.com/spreadsheets...?usp=drive_web
Thanks for that--I figured it was a 223 case but with the bullet seated way out to the lands--I found some data on it but it was with a 26" barrel and cartridge OL well past anything that would fit in a conventional AR. I looked at the numbers in the table you supplied for a 95 gr bullet (fed fusion) but this 2800+ fps with 25.9 grs of benchmark has me kinda stumped--what kind of pressure does that run at?

PS: I noticed the barrel was something called 8T AR--is that literally an 8 groove rifling (I noticed MDWS offers 6 groove 20" barrels)?
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Old May 16, 2017, 08:13 AM   #8
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Originally Posted by stagpanther
PS: I noticed the barrel was something called 8T AR--is that literally an 8 groove rifling (I noticed MDWS offers 6 groove 20" barrels)?
1:8 twist, and it's the same pressure as 223. Starting point is to use 6x45 data, but with the extra case volume max is usually 1/2 to 1 grain higher.
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Old May 16, 2017, 11:00 AM   #9
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1:8 twist, and it's the same pressure as 223. Starting point is to use 6x45 data, but with the extra case volume max is usually 1/2 to 1 grain higher.
Taking the 6 x 45 and increasing case H2O capacity by a full grain to 32.5 and allowing the bullet to be run out to 2.285--QL is telling me that the 95 bst --and 25.9 gr of benchmark is a highly compressed charge boosting pressures well past 70000 psi.
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Old May 16, 2017, 12:11 PM   #10
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Postman delivered my barrel and bolt this morning--so here is the completed rifle:



I decided to "mix things up" a bit and change the look from all my other AR's and use a battlelink minimalist buttstock instead of magpul--seems cool but doesn't slide near as easy as a magpul does on the tube--but the flip side is there's no annoying wobble, either. Other than that just another AR build, notable pieces being a wilson combat adjustable gas block, PRI combat latch, NIB carrier for the 750 extreme duty bolt and a jard adjustable 2 stage trigger. I didn't have a spare tactical scope lying around, so I had to press a simply loopy 3 x 9 rifleman scope into service--should be adequate for getting on paper and initial load development.

There's definitely going to be some cavity back bullets and magazines in the future for this gun, the chamber was designed to allow for long seating of bullets past the 2.26 typical limitation so I should have some fun developing interesting loads.
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Old May 16, 2017, 01:43 PM   #11
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Buy some ASC magazines, you can have a COAL of 2.31" with them.
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Old May 16, 2017, 01:47 PM   #12
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Love this thread!! Like a bunch of tuners tweaking a GTR These custom rounds are really interesting. Someday I would love to take this up, but it clearly takes a lot of time and expertise.
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Old May 16, 2017, 04:00 PM   #13
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Quote:
Buy some ASC magazines, you can have a COAL of 2.31" with them.
I have a couple already--the cavity backs will let you run 2.336.
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Old May 16, 2017, 05:19 PM   #14
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Spent about 3 hours breaking the new rig in and dialing in the scope to get on paper, overall the gun shoots great, couple of minor adjustments still need to be made but I think I have a winner.

A couple of hick-ups along the way--the first was that I discovered the lock screw on the wilson adjustable gas block is missing--I never bothered looking at it closely until I put the gun together today. It may have simply fallen out--but basically I didn't touch the thing until today. Imagine it shouldn't be hard to find one at a hardware store.

Primary mission was simply to break the barrel in and clean the bore after each shot initially, then after every 3 shot group. Needless to say, this wasn't a pursuit of developing loads for accuracy--just seeing if everything worked fine--which they did..for the most part. The resized grendel cartridges chambered and fired without a hitch, and even though I was stopping to clean and the wind was blasting the target--I still got a couple of decent groups considering.





The second hick-up came as I shot the last group of warmer loads though still well under the max rating for the cartridge. The first shot blew the primer out, and I noticed that the second cycled cartridge was jammed in the chamber--upon extraction with the handle the case came out sans bullet. Since all the loads were of the same bullet at the same COL this seemed a bit odd, but then I noticed the BCG was dripping with cleaner/lubricant a bit excessively, my guess is that although I thought I swabbed the bore and receiver enough after each cleaning, I probably had too much remaining which then seeped into the chamber and caused the bullet to jam short in the chamber. This also points to the need for me to make a modified case out of one of the spent cases so I can be sure exactly where each bullet engages the lands (or throat) with a comparator.

post-edit--I've since confirmed with a chamber ogive measurement that the ogive on the BST was hitting the throat and setting the bullet back a bit, this resulted in a slightly compressed charge on the top-end load producing just enough pressure to blow that primer out. Also, apparently the design of the hornady grendel brass is such that it readily bulges at the head when going much past about 60K psi; actually a good thing when you think about it as other case designs don't necessarily do that and might conceal wandering into an over-pressure situation.
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Old May 16, 2017, 07:05 PM   #15
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That first group is nice!
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Old May 16, 2017, 07:43 PM   #16
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Thanks--I'm not really trying to get good groups--but I still tried to take good aim with each shot. Going to resize some grendel brass to predator and then tap one to make a homemade modified case--then get down to some serious load development.

Interesting thing I thought the 20" barrel would push the overall weight close to AR10 territory, but my scale sales it weighs 8 lbs decked out--hefty for an AR 15 for sure but still reasonable for long distance humps through the woods in my book.
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Old May 17, 2017, 05:46 PM   #17
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"built" a modified case today for the 6 predator--took me a couple of hours but I got lands/ throat engagement measurements for all the bullet types I have. In retrospect--I should have done this before firing off the first loads above. Wildcat chambers may or may not conform to the range of bullets--I should not have assumed the maximum length that would fit in the magazine, or even the "standard" 2.26 would necessarily chamber without engaging the lands or throat and potentially create a bullet set-back.

That was not a smart thing for me to have done.

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Old May 17, 2017, 05:59 PM   #18
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Love this thread!! Like a bunch of tuners tweaking a GTR These custom rounds are really interesting. Someday I would love to take this up, but it clearly takes a lot of time and expertise.
Hmmm..I'd venture to say that it if I can do it--a cave man could; the only thing is that you have to be thorough in the steps and when in doubt--ask an expert (which I am not). Most of the wildcat manufacturers are very small outfits that have a high vested interest in helping their cartridges catch on; so they generally tend to be very helpful when calling for advice.
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Old May 17, 2017, 07:29 PM   #19
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"Most of the wildcat manufacturers are very small outfits that have a high vested interest in helping their cartridges catch on; so they generally tend to be very helpful when calling for advice."

That describes Harrison at AR15 Performance to the tee! The wildcats do require the time and effort. I've only done one (270AR) and not sure I'm up for another due to the limited amount of shooting I seem to be able to do of late.
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Old May 17, 2017, 07:46 PM   #20
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That describes Harrison at AR15 Performance to the tee!
Yup--he's a perfect example. I've been eye-balling his upcoming 30 Herret--and although I already have a 5 xx 308 builds--that 270-08 barrel looks MIGHTY interesting
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Old May 18, 2017, 05:46 PM   #21
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After I measured all the bullet engagements to lands/throat my hunch was confirmed--certain bullets with their "fat" parts of the ogive forward prevent them from being seated at a SAAMI "standard" of 2.26 for a comparable grendel cartridge (and even a few of those are seated shorter depending upon the manufacturer.) Harrison at ARP specifically designed the chamber to handle high BC/SD bullets with "longish" tapering ogives--like the ones commonly found on berger bullets and matchkings. My first crack at a wildcat--I'm learning why they are called that.

Here is my first group of reloads for development using berger's 95 gr VLD. Unfortunately, I forgot to get their hybrid VLDs specifically made for AR's--their regular ones often have feeding issues in AR's of any caliber in my experience. Hopefully it will work out.

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Old May 19, 2017, 12:24 PM   #22
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Today's test shooting went very well for the most part--the biggest problem was the weather--two days ago it was about 40 degrees and wet , and today it's in the mid 80's, very dry and very unstable--gusty winds into 30+ with dust devils swirling about, my target stand with about 50 of weight holding it down blew over. Felt more like the Mojave desert than an island in the North Atlantic! Besides that, I simply wasn't shooting well, so those are my excuses and I'm sticking to them. I'm still breaking the barrel in and stopping to swab the bore out, these are cartridges 25 - 50 so I'm certain the best is yet to come.

The VLD's managed to cycle without a problem, though I did check them a couple of times after being chambered and they show just a "smidgen" of tip deformation, I'm guessing they are glancing off the chamber face rim on the way in. No pressure signs of any kind through out the range of powder charges--I did have one short stroke in the lower range but that went away as soon as I opened the wilson gas block a touch. The jard trigger is working great, though the break is just a tad gritty for my tastes so I'm taking it back apart and see if I can smooth that off a bit



Here's the first group out of the cold bore--excluding the flier at lower left it would have been a pretty decent group had I shot better. All the rest of the charge ladder grouped in the 1 MOA range +/-, not all that remarkable normally but I'm extremely satisfied given the conditions and the fact that my game was off. Also interesting was the fact that all the groups held together well regardless of the powder charge weight



Next up will be some 85 TTSX's which should be zipping along at around 3000 fps and 1700 ftlbs energy at the muzzle.
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Old May 20, 2017, 06:15 AM   #23
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Took the trigger group apart and did a little adjusting--it now breaks as smooth and light as a butterfly's wing. Jard adjustable triggers remain my over-all favorites for AR builds at their price point. Some people don't like the pistol-grip hole sear adjustment arrangement, but I do like it because the sear engagement is based on the trigger hitting a big steel screw--other designs that rely on the small adjustment screw drilled into the trigger itself I've found, over time, can dig "divots" into the floor of the firecontrol well, which can in turn throw off the timing of the trigger. The only drawback to this design is that the lower's pistol grip hole needs to be tapped all the way through the receiver--but that's easy to do in just a few minutes with a common tap kit

I hope this helps today when I try some "secondary" 95 vld loads. It's still going to be windy, but hopefully not nearly as bad as yesterday was.
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Old May 20, 2017, 12:41 PM   #24
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Just back from the range--despite a forecast of diminishing winds--it was still blasting pretty strong with gusts to 20+; however after the adjustments the gun is humming like a Stradivarius.

Had a couple more CCI #41 primers pop out today; I again checked cartridges that had been cycled through and I'm still getting some deflection of the tips off the rim face on the way in--I believe causing some bullet set back. I believe my reasoning is correct because the primers popped on mid-range loads that were loaded from the magazine by the normal cycling--but when I simply dropped the bolt on a cartridge that was positioned in the chamber; using very warm top end loads--there were no popped primers or other signs of over-pressure.

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