AR Sponsor
Posted: 12/17/2006 4:32:56 PM EDT
|
I know why it is taboo to span across a flat upper to a quad with a scope, mount or what ever.... My mid length RRA with a DD rail looks perfect, but I put a machinist level on the rail and to the upper flat top and WOW!! Out of clock (angular) and I can image when the gun starts to heat and cool... 77Bronc |
There are tricks to aligning the rails, mainly useing a removable carry handle, riser or the Brownells tool. Even with the rails aligned, you should not mount a scope or any optic sights on the top rail of a quad rail system that's attached to the receiver unless you span both the receiver and handguard system with a rail first. |
Randall, I've heard this for a long time, but can you elaborate on the specific reasons why? I would imagine that it's because the reciever and quad rail can drift relative to each other upon heating and cooling, thus screwing up your zero. But I'm just guessing.. and is a full rail spanning both going to be just enough to prevent it, as opposed to a smaller clamp spanning both such as an EOTech mount? Thanks- A_J |
| I'm not answering for Randall, but Mark LaRue is against "bridging" because he says the Picatinny rails from the two different sources are mostly likely different in dimension. This will not allow the mount to sit securely. Tight on one rail, loose on the other, but never tight on both rails at the same time. |
The railed handguards can move relative to the receiver without even heating and cooling them. That just makes it worse. It's both an issue of wandering zero and damage to the scope tube from the rings moving around. |
That's absolutely true of Mark's system which is a single-width clamping setup. I'm not a big fan of ARMS, but they did it right. Seperate mounts in front and in back solve for this problem. Look at the ARMS #38 SPR-MOD. Clamps to the receiver and screws into the PRI handguard. It's a big heavy chunk of solid rail that you CAN put a scope forward of the receiver with. Otherwise, the Swan Sleeve or LaRue riser will reach out over the handguards while only mounting to the receiver, which is the right way to do it if you don't bridge the handguard to the receiver with a rail first. Of course the best way is to just get an MRP/VIS/LaRue Monolith... |
|
Thanks, Randall. I've got a Troy 13" on a carbine, and I did notice that all the way out at the muzzle end, the barrel is off-center (low) by a few hundredths. I did use a rail to align them, but upon closer examination of my upper, I found that the barrel nut flange isn't perfectly square to the top rail - get all the way out to the muzzle end, and this is magnified. Visually it looks fine though, but get a mic and a straightedge on it and you can see where it's a hair off. Makes me wonder about the zero on the front irons drifting too.. |
AR Sponsor