Short COAL 129gr Interlock SP - Safe to Fire?

[Nephilim7]

First time Grendel owner being guided through starting hand reloading by someone else with no Grendel experience. Working with a home built AR with a new barrel from a reputable manufacturer. I know and understand I have started with a "potbellied" bullet thanks to the ogive of this heavy projectile. This may be the root of my problem. I have loaded some loads and want to poll those that know:


(1) Is the load below safe to fire with such a short COAL?


(2) Is the developed COAL short enough I should be concerned for the reamed dimensions of the chamber, being an unfired barrel? I have no gages. Should I speak to the supplier about it?


Unfired, virgin Starline brass fits in chamber and extracts fine.

I see on the forum how an AR Grendel load for Hornady Interlock is expected COAL=2.245" but I can't come close to that and lock bolt lugs. Then extraction leaves the bullet in the barrel, dumping powder. I seated deeper and deeper until the round fit in with a firm fingertip press of the bolt carrier rear to close and lock lugs. Hand extraction by charging handle is now easy and smooth.

Concern: COAL is 2.131" - way off from what I expected. Nearly bottomed out the dye bullet seat adjustment.

Powder can be heard shaking just a little inside the round, so powder is not compressed. No crimp yet but doesn't seem to be the problem. Load data below.

Starline brass (virgin)
Hornady 129gr Interlock SP
25.0 gr AR Comp
CCI 450
Case trim length: 1.510"
COAL: 2.131"

I had to go this short to close the bolt and not pull the bullet on extraction. Again, no neck crimp yet.

Thanks for your help!

Neph

[VASCAR2]

The only load data for AR Comp I’ve found is Speer. Speer shows a maximum charge of 25.3 grains of AR Comp with the 120 Grain Gold Dot. Speer shows a maximum charge of 24.4 grains compressed charge with a 140 grain Hot Core SP. The 120 grain Gold Dot is a bonded core bullet that build pressures faster than a typical cup and core bullet.

It appears the chamber is on the short side seeing how your having to seat the bullet deep. In new unfired brass in a brand new barrel I would probably use a lower starting charge. Speer shows a minimum charge of 22.8 grains AR Comp with the 120 grain Gold Dot. I think I’d try 24.0 grains and go up in .3 grain increments. Your in uncharted territory with a new build and I’d use a chronograph while working up.

I think AR Comp is a very desirable powder for the 6.5 Grendel but there is very limited published verified pressure test data for AR Comp. Speer shows a C.O.A.L. of 2.140” with the 140 grain hot core bullet. I think the SP profile is similar to the Hornady SP so the main difference is a longer bullet body of the Speer. With the small case capacity of the 6.5 Grendel longer bullets impede how much powder you can get into the 6.5 Grendel. New brass supposedly has less case capacity compared to fired brass and is another reason I’d start with a lower charge. I suspect 25.0 grains of AR Comp will be close to a max load with a 129 grain InterLock SP.

[grayfox]

I also think you've got a short chamber, seating in to 2.13 +/- is much shorter than Hornady lists for the coal of the 129 sp.
They list oal as 2.245. So you could definitely be into unknown pressure spikes at the load you're trying to start with.
Grendel is hard to model in my Grt, which I use for advice... and AR comp is a slightly faster version of RE-15 according to Alliant...
IMO you would be better advised to begin with a ball powder like BLC2, AA2520, or CFE223 and work up from the low starting point in the Hornady load data.
My starline brass that I have is pretty close to Hornady in volume etc, so that could be basically a wash if you start from the low.
And definitely use a chrono because especially in your case, without it you are driving blind down a foggy windy road in the country.

[grayfox]

ps, if you don't have any of those powders, you ought to consider getting some factory ammo to at least start in this new barrel.
Look on ammoseek for some to get.
In handloading as frankly in all shooting topics, safety-first is the paramount principle.

[grendelnubi]

Using a 2.245 COAL with the 129 SP would put it close to the lands in my Odin Works barrel. I run 2.2185 COAL with 26.9 grains of AR Comp in two of without issues using Hornady brass which based on my measurements is a little smaller than Starline brass.

Is this a bolt or AR gun? This is important information, however based on my experience, using your 2.131 shorter coal will increase pressure but your starting AR Comp 25 grain load with Starline brass you should in the safe zone. I believe you will find something north 26 grains of AR Comp will work for you. More if this is a bolt gun.

My Odin is about .040 shorter chamber than my Faxon barrel so based on this, I would consider my Odin to be a little on the short side. Your chamber based on the details you gave is about .090 shorter than my Odin. You may consider calling the barrel manufacture to discuss the chamber size is if you think it will be an issue.

[dpete]

Using a 2.245 COAL with the 129 SP would put it close to the lands in my Odin Works barrel. I run 2.2185 COAL

I too have an Odin works barrel and run the 129 SP at 2.220 in RP 7.62X39 brass I resized then fireformed with the rifle. The COAL is where they shot the best.

[lazyengineer]

You're fine. Yes, that COAL is a bit low, but I find Grendel chambers to often be quite short throated, so am not surprised. In general rifle compressed COAL isn't a big deal like pistol round COAL is. Rifle powder is slower burning and the reaction chamber volume is much larger. The relative decrease in initial reaction chamber volume compared to a pistol casing of .1" COAL shorter, is much lower by percentage. And peak pressure in a rifle doesn't usually hit until about 2" or so of travel, unlike a pistol which hits it darned near right away at about 0.1" even, by some analysis at least.

Point being, you will be safer running COAL, than you will being in the lands, which does give a pressure spike, since the bullet can't start moving to decrease pressure any at all, until there's already enough force under it to start deforming it into the lands. Gunpowder burns in a bootstrap fashion, where the higher pressure the faster it burns the higher the pressure. In too short COAL, a rifle will start pushing the bullet forward and increasing the reaction chamber size (and thus decreasing gas pressure growth rate) relatively easily and at lower P.

Also, 25 gr of ARComp isn't an excessive charge. I run 26 gr under 123 bullets, though will admit that might be overly strong.

[ported45]

I found Hornady's recommended COAL to be too long in both chambers that I have loaded for. One is my own Ruger Predator that I know does not have a short chamber.

[EastTNHunter]

With that bullet I have to seat my coal pretty short for both my AR and my Howa mini. Shoots quite well

[rickt300]

The Hornady bullet has a full diameter pretty far out on the bullet so it will often have to be seated pretty short. That said just start out with the listed starting load and work up. Is the cannelure in front of the case mouth? In my Grendel it is not.