ar15/xcr/acr/scar carrier rails

[kal]

I have noticed that all 4 designs have an aluminum receiver.

Except, the ar15 and xcr have the bolt carrier riding on bare aluminum. The acr/scar have steel rail inserts for the carrier to travel on.

Why is this?

[MisterWilson]

I have absolutely no idea, but if I had to venture a guess I'd think it would have something to do with the symmetry of the carrier, that perhaps one design places more canting/lateral force on the carrier.

Tag for better answers...


ETA: Tilt. That was the word I was looking for...

[variablebinary]

The AR15 and XCR dont need steel inserts.

The AR15 and XCR dont have oprods tapping the carrier face, so the BCG doesn't experience any lift or tilt. On the XCR gas directly impinges on the piston head and rod, blowing it straight back, so you dont get funky uneven wear or gouging of the upper.

Also, the AR15 and XCR have serial numbers on the lower. The upper is a throw away piece; disposable and easily replaceable

The ACR and SCAR have serial numbers on the upper. If the upper experiences any wear or damage, the owner is screwed. Bet your ass Remington and FN better take ever measure possible to ensure the upper gets as little wear as possible.

At any rate, we got FN and Magpul heads on here, so maybe they can chime on the design philosophy behind the internal rails

[Jaws]

The AR15 and XCR dont need steel inserts.



Also, the AR15 and XCR have serial numbers on the lower. The upper is a throw away piece; disposable and easily replaceable
The ACR and SCAR have serial numbers on the upper. If the upper experiences any wear or damage, the owner is screwed. Bet your ass Remington and FN better take ever measure possible to ensure the upper gets as little wear as possible.

At any rate, we got FN and Magpul heads on here, so maybe they can chime on the design philosophy behind the internal rails


I'd rather replace the fifty bucks rails, than 1000 dollars "disposable upper receiver". :D

[kal]

I'd rather replace the fifty bucks rails, than 1000 dollars "disposable upper receiver". :D

Fortunately, you'd probably ravage most other parts of the rifle EXCEPT the steel rails.

[JoshNC]

SCAR bolt carrier rides on aluminum rails, not steel. The raceway for the cam pin features a steel rail above the aluminum bolt rail.

[olds442tyguy]

Also, the AR15 and XCR have serial numbers on the lower. The upper is a throw away piece; disposable and easily replaceable


If caliber conversions alone cost as much as an AR15 upper, I can't imagine many people just throwing them away and counting out their pocket change for a complete new XCR upper. I sure as heck don't consider an AR upper as being disposable though. That's just me though. :D

Being able to use an alternate platform's magazines for a common non 5.56 caliber proves that the street goes both ways too. For the cost of twenty 7.62x39 AR15 mags you could have a new lower and use cheaper and less finnicky original AK magazines. The same goes for 5.54x39.


I'm guessing they have steel rails simply because it's better. If you're going to have the carrier rails be separate from the receiver shell, why bother with aluminum? I'm guessing the rails will be one of the last things to wear out too, but choosing steel just adds to the longevity.

[Todd.K]

On the SCAR the carrier rails are not separate from the receiver. There is a steel insert where the cam pin rides.

[variablebinary]

If caliber conversions alone cost as much as an AR15 upper, I can't imagine many people just throwing them away and counting out their pocket change for a complete new XCR upper. I sure as heck don't consider an AR upper as being disposable though. That's just me though. :D



In the short term, with low use, low round count uppers its fairly meaningless.

In the long term, the upper takes nearly all the wear with use.

Lowers can be mated to new uppers very easily and you're back in business without having to purchase a new firearm.