The 13" AX rail in front of me is 13oz on the nose. The ID is 1.410"; the outside dimension is 1.648" across the flats (the outside profile is octagonal, of course). If you're trying to estimate the weight based upon the dimensions, keep in mind that actual rail length is about 1" longer than the nominal length, and there is a 4" section of the extrusion that is much heavier profile in order to provide the abutment for mounting to the chassis.
Well... Shit. I'm not sure it's worth it at 13oz factory.
There are three "problems" with AI's factory rail:
1. It's not Keymod. It's stupid AI "KeySlot" which ironically, they are now trying to patent with exclusivity to KeyMod. But with the common use and common application of KeyMod, they're shit out of luck imo. It would be nearly impossible for AI to fight all the KeyMod mfgs. It seems in PSR and newer they have started putting a chamber (or at least a different chamfer) inside each slot like KeyMod uses. Even more proof they are being foolish trying to patent, as this is a feature KeyMod had already. For some of us with other Keymod guns, it makes little sense to carry two very similar but incompatible mounting systems. It would be great if AI got their head straight and got on board. Even if you aren't concerned about other systems or easy swapping, you're locked into AI's prices for attachments. I don't like vendor lock-in.
2. The scope is mounted a little high. Not that it matters on these guns, it's adjustable comb. But, if one wanted to swap optics over to another gun, the optic from the AX platform wouldn't really fit on many others unless they too had full rails or over barrel mounts. It seems most of the people with AX chassis are not on a budget however, so this likely isn't all that big of a deal.
3. I'm curious how the 13" is 13oz because I guessed it to be around 18+. I'll draw it up again today with E Bryants numbers (thanks very much byw!). Still, the AX chassis really does seem like it should be lighter than 5.5-5.75lbs! I'm curious where it's so heavy. Anyway, I WAS looking at doing a carbon fiber forearm.
4. The attachment on the pre-2014 rail is flat out stupid. Clearly, because they fixed this for 2014+
5. There isn't much of a shooting of a barricade surface with the AX.
I had been working... well, that's not correct, I had been toying with a composite forearm for another company's product. I came up with a nice design after some poor attempts and becoming familiar with the product. But the more I worked on it, the more I realized some aspects just aren't "quite there yet" on the base part. I look forward to seeing what that company does in another generation or two, but right now the 2014 AX chassis is more ideal for me. I'll come back to that other company, but in the meantime, it also made no sense to give them my carbon design surfacing, considering the lack of information I could get out of them in return.
On the AX however.... I drafted up a contoured carbon tube (overall round, but at no cross section it was a circle) that attached at the same points, held pic or keymod sections, avoided carbon to metal contact directly into the fibers, and even figured a cheap way to mfg it in one layup of pre-preg using ren plank for molds and not aluminum to prototype. But....... At 13oz, and equal (probably superior) strength carbon/aramid part would be figure no less than 6oz when the attachment sections were put in, doesn't make sense for my time investment.
I'll probably go with a Plan B, which would still save some weight and be pretty trick. I get all the solutions I wanted except for the contoured look and it'll likely cost me less. If the AX overall-weight becomes an issue for the role this gun is filling. I'll probably see what is on the market next year. Other mfgs have some nice options that are close and should only get closer in the future. APO, KRG, etc
... I'm still going to draft up a model to see that weight.
Thanks!
