Wow, they have a 15" model now? Does this one have the revised design that works with standard barrel nuts?
I have a 7" Troy MRF and I wanted to go with the 13.8" Troy MRF-RX, but now that they have the brand new 15" TRX in the stores… I am going to go that route instead. So make mine a 15" TRX.
O.P. I would go with the longer of the two that you mentioned. I would try the 13" if I were you.
No, That is the new Troy Alpha Rail. The Alpha is different than the TRX Extreme Rail.
You can get away with a lot depending on where you like your stock. On my work rifle I have a 9" rail on a carbine length 10.3" MK18. On several of my personal rifles I have various rails even the 13" Troy. I can get the same extension on any of these rifles with adjusting the stock. On my work rifle stock fully extended gives the same LOP as my personal 13" rail with stock almost fully collapsed. Now some might have preferences on where your optic sits but rail space gives options for that also.
Here’s the link for the 15" Extreme:
http://troyind.com/%20/battlerails/troy-battlerail-trx-extreme-15
Wolf is right, the 15" extreme has the same locking system as the 11" & 13" models. The alpha uses a standard barrel nut, while the extreme uses a proprietary. Either way, I’d be interested to see just how durable both of them are.
Glad I this threat come up. I’ve been debating this same question the last fews weeks myself only I want to put the rail on a 14.5 middy with perm attached Battle Comp…decisions decisions.
Oh… thanks for clearing that up. Looks good though:
http://troyind.com/%20/battlerails/troy-battlerail-alpha-coming-soon
One problem with the Alpha, though, is that the built-in spring-loaded battlesight sort of precludes the option of a 12 o’clock light mount. 
Will the 11" TRX cover a LP gas block on a midlength?
Yes no problem.
Fit over the short LPs just enough to hide them. I am using a Troy gas block and the 11" goes past it by about 3/4".
I bought the 11" and ended up trading for the 13". I’ve since sold the upper. I liked the 11" a little better because it was closer to the carbine length guns I am used to. It is really preference and something you will have to try out. I was quickly able to trade my 11 for a 13 here on the EE so that is always a good option if you want to experiment.
I also found it a little more natural to operate my light on the 11" rail. I’m 6’.
Here is how it ended up when I was finished.
This is a comparison of my SBR’s and the middy. The middy upper’s been sold I might SBR the spare lower one of these days.
I use a 13in TRX Extreme on my 3gun BCM 16in Middy SS410.
When I get my 12.5 SBR done, its going to be wearing a 11in TRX Extreme
“The genesis of modular free float rails has arrived. Building off of the TRX™ Extreme design that revolutionized rail based handguards; the Alpha Rail™ utilizes a new low-profile locking mechanism, which offers unparalleled strength and stability. With built-in legendary Troy™ spring loaded front Battlesight, unlimited mounting options, and greater heat dispersion the Alpha Rail™ once again sets a new industry standard in modular free float rail design.”
Based on this description I am curious if they will be similar in weight to a TRX with a BUIS.
I am in the market for a new rail as well. If i decide to replace my FSP with a LP gas block I will probably be looking at a 11" or 13" TRX. This new Alfa rail has me interested now. I would love to see some specs.
I just did a 12" 6.8 upper and used a Wilson Combat barrel. The 11" TRX VTAC stopped just short of the 5/8-24 threading so a 12.5" will look really good.
Once the 15" version comes in the VTAC flavor, I’m definitely going to be picking that up.
So I assume that if I’m going to shave my FSB on a middy I should go for the 13"?
Hayden,
The 11" rail will cover a mid length gas block no problem.
A shaved FSB is longer than a normal gas block. Don’t know how that affects anything.
Read elsewhere that a 11" will cover a shaved FSB on a middy.
I do understand what you are posting, but “LOP” refers to length of pull – the measurement, in inches, between the curve of the trigger and the edge of the butt stock. Collapsing the stock does change this measurement.