Home Forums Hunting & Shooting AR and Tactical Rifles 6mm AR or Fat Rat

A friend of mine just gave me a Krieger AR15 blank that is 6mm with 1-8" twist and rifle length gas. For those shooting the 6mm AR or 6mm Fat Rat, is the rifle length gas system going to tie me to the 6mm AR? I had planned on the Fat Rat with +2" gas but now I have this barrel.

I see no reason why rifle length gas would tie you to the 6mmAR vs the Fat Rat.

One thing that can matter relative to gas length is barrel length.

How long is your new Krieger?

It's a 27" blank so I can cut it to whatever length I want. Usually about 8" from the port is a good length for the crown so in this case I am really looking at about 22" but think it could be a 24" barrel. I do think that port location has something to do with what powders you use and that has something to do with which cartridge I would pick. Everyone I know that uses a Fat Rat shoots +2" and everyone I know with a 243 LBC shoots a rifle length system (because they are all BHW barrels). Guys who shoot the 6mm AR often get +2" when shooting 24" or longer. So, I guess what I'm asking is, "Are there any guys here who actually have a Fat Rat with rifle length gas? If so, would you recommend it or would it be better to shoot the 243 LBC (since it's less work) since I'm limited to rifle length gas?"

There isn't much difference in case capacity and resulting port pressure between the 2. It isn't like going from a 22 cal to a 30 cal where the pressure drops so much. The main thing is to drill the gas port the correct size or use a good adjustable to limit the gas flow to the carrier. Personally I wouldn't use a fat rat and fireform cases to gain that little amount of velocity.

Agreed, that was my main reason for going with the 243 LBC instead (BHW's version of the 6mm AR). Forming brass from 6.5 Grendel is no different than just resizing fired brass for this one, it's just a straight neck-down with no shoulder or body changes. My accuracy load does 2920 fps with the 95gr Berger VLD (24" barrel, rifle gas), although others like Smack on this board have reported 3000+ fps with the same bullet.

With the 6x6.8 SPC and the .243 LBC, CFE223 is the powder to go with. Spooky accurate with 95gr VLD's and 105 A-Max's and gives top speeds. I was breaking 3000 fps with a 95gr Berger hunting VLD and it was shooting 1/2 moa. The A-Max's shot shot really well at 300 yards with CFE223 powder.


I have been shooting and assembling AR's for many years! I can't even recal all the chamberings! By far the .243 LBC has been the most accurate long range AR15 chambering I have ever shot! Increadably accurate.

This is an interesting thread for me, as I have not built a 6/6.5 rifle yet & the 243 LBC sounds like a winner.

Is there any need for any special consideration for the upper to be used for that round? I'd likely use a Viltor.

MM

Nothing special about the upper MM, just a different barrel, bolt, and mags. Everything else can be standard AR15 stuff, whatever your preference may be.

Thanks, Yondering, that's what I thought.

What is your opinion on using a standard 7.62x39 bolt with the longer firing pin vs using one of the Young's bolt that accepts a standard length firing pin & supposedly beefed up lugs?

Is Black Hole the only source for that chambering, as I've not seen it elsewhere & I know that BHW designed the reamer for it.

Their lead time is generally pretty long, but since this would be a winter project, it really doesn't matter too much.

MM

Young was out of bolts when I built my 243 LBC, so I'm using a good quality "standard" 7.62x39 bolt with a standard firing pin. I'm not sure why the pin would need to be longer? That's the first I've heard of that.

243 LBC is BHW's name for their version of the 6mm Grendel-based chambering, so you'll only find it from them. If you check their website (I believe the sales site is called "True Sporting Arms" now) they do sometimes have some 243 LBC barrels in stock; if you're lucky they might have what you want. I only waited 4 or 5 weeks for mine, but a buddy who ordered about the same time waited almost 3 months for a 6.5 Creedmoor barrel; that was early this year before they kept anything in stock. Both of our barrels shoot really well.

Yondering, thanks for the info.

If I can't get a 243LBC barrel in a reasonable amount of time, I may just do a standard 6.5 Grendel first & then do the 6mm when I can get the parts.

MM

I feel like SMACK said he was using neck turned brass in that thread with the 243 LBC. Was it due to chamber dimensions, donut prevention, or what? I'm still debating what to do with this barrel cert and a 243 LBC is right on the heals of a 6x47mm Lapua. I'd probably only use it for matches I drove to but I also don't want to put the time into neck turning when I lose a lot of brass (30-40%) at matches if I shoot a gas gun.

I don't know what Smack is using, but I'm using straight Lapua 6.5 Grendel brass in my 243 LBC barrel with no neck turning. I've also tried forming from 7.62x39 brass and didn't need to neck turn for that either, at least in regards to chamber neck dimensions.

That's with BHW's 243 LBC chamber; with other variations of this cartridge you may or may not need to neck turn. If you were to chamber one of these yourself with a 6 PPC reamer, you'd end up with something that required neck turning and thin necks in most cases.

The cert is for BHW and I've decided I'm doing something different for a 6.5 Creed so I'm going to get a 243 or 264 LBC. I don't like nor being able to buy ammo but I can use it for matches I drive to and fly with the big gun. The 243 LBC looks like a better round for LR gas gun matches, it just means I would need to download a bunch of 123gn SMK I won't use unless I have a Grendel.

Lapua brass in both mine and my fathers 6AR ARP barrels without a single issue. 95gr sst’s at 2925 fps shoot stupid good.

I settled on Hornady brass for my .243 LBC & .264 LBC chamberings. No neck turning and they both shoot well. I got tired of looking for and loosing Lapua cases when shooting fur or hair, especially in the snow.