Feed ramp/ lugs question

[Bretandry]

Hello folks.
What is your personal practice on a new drop in barrels feed ramps and lug area polishing and smoothing? Is there a proven practice on the Grendel due to it's larger case diameter? Are there areas to pay more attention to for irregularities? Thanks for any advice in advance.

[VASCAR2]

I’ve never had an issue with any of my builds where they needed any attention. Shooting rifles and cycling the brass through the new barrels or magazines seem to wear the sharp edges smooth. The brass gets scratched up but I’ve never seen any long term negative effects on my brass.


LRRP52 is the person to chime in here.

[603 Country]

I just had a problem with my 223 AR, which might be of interest. For years I’ve shot 40, 63, 55, and 65 gr bullets, all reloads, through the rifle with no issues. I didn’t even crimp the reloads. And then I decided (partly because they were available) to switch to Nosler 64 gr Bonded Solid Base bullets. I measured the chamber, as I always did for bolt guns, and although Nosler’s tested load was at 2.175”, it seemed that my load should be 2.215”. I loaded up a bunch at the longer COAL. Every now and then the rifle would jam up when a bullet was shoved back into the case. Hmmm. I thought maybe the feed ramp was the problem, or maybe the mags, but the actual problem was that I had loaded that blunt nosed bullet just right for the chamber, but a hair too long for the action. The longer round was bumping something. I went to the Nosler tested length and all is well, and I found a very accurate load at that shorter COAL. I do use the Lee Factory Crimp Die on the 64 gr reloads now.

[LRRPF52]

The M4 feed ramps are part of the solution to feeding 6.5 Grendel reliably. M4 ramps are lowered relative to the bore axis, which allows the fatter case to index correctly.

The feeding cone in the breech is also a critical geometry that needs to be machined right, otherwise you will get the situation with larger meplat bullets like 603 Country did with Nosler 64gr .223 Rem.

There are a lot of things that have to come together, starting with feed lip geometry, magazine index height relative to the bore axis (controlled by the placement of the magazine catch hole in the lower receiver), then the placement of the feed ramps, their angle, their height, and then the feed cone in the breech of the barrel.

This really turns into an argument for a competent manufacturer who has worked all these issues out and established their own TDP for Grendel or whatever cartridge they’re chambering, vs the DIY/parts market.

As DIY’ers, we tend to want to assume that all we need to do is Lego some parts together with the best of intentions, entitled to the stars aligning for us, and expecting it all to work out.

This expectation is especially optimistic when we buy bottom-barrel parts.

For my personal approach to this, I look for barrels from companies that have been around for a long time and have a reputation for reliability #1, followed by accuracy.

My first barrel/bolt combo was from Alexander Arms.

My 2nd was from Lilja with the 1st Group Buy we did here.

I don’t want to change the angle of the feed ramps or mess with that geometry, but just take the sharp edges off the teeth along the sides of the ramps and polish the ramps so the coefficient of friction is reduced and especially my BTHP meplats aren’t damaged as much.

One problem I have seen is that when fitting the barrel extension to the upper receiver, the bottom edge of the ramps can sometimes be proud of the corresponding M4 ramps in the upper receiver.

For those who are unaware, the AR15/M16 family never had extended feed ramps in the upper until the Colt Model 727 (M16A2 Carbine), which then were carried over to the M4.

Reason for that was on AUTO or BURST fire, the action would start to out-run the spring rate of the magazines, and cartridges would be prematurely fed before they could lift into feed lip presentation, so they would hit the flat face of the upper receiver right under the barrel extension.

The serendipity of that for 6.5 Grendel is that the larger, lower feed ramps of both the upper receiver and barrel extension played well with the fatter cartridge.

These aspects of the AR15 have been constantly looked at throughout the life of the program by DoD, gaining more scrutiny as more carbines were fielded in the 1990s, with most of the emphasis on magazines.

Suppressed carbine combos are the hardest to get to run reliably.

[LRRPF52]

Another issue is that many bolts aren’t machined with the correct dimensions for the barrel extension teeth clearance, so you can run into a situation where lugs are grinding against the teeth as the bolt telescopes in and out through the extension once it’s unlocked or returning to battery. We’ve seen a lot of this on 5.56 and .308 rifles where a DIY approach, or even manufactured rifle, have interference between the lugs and teeth.

Some of that is caused by barrel extensions not being timed correctly relative to the upper receiver. This is controlled by the index pin placement and perpendicular installation to the tangent of the extension, as well as how the installer torqued the barrel nut into place.

I avoid this problem by checking the fit and articulation in-advance, then setting the barrel extension timing by bedding it well before I ever torque the barrel nut on, so there is no way for the extension to wiggle out of place.

Basically, the whole way I approach an AR-15 build today resembles nothing how I did it back in the 1990s-2000s. It doesn’t even feel like working on the same gun.

[Bretandry]

Thank you sir...Got it a standard polish job on the ramps and the corresponding edges with attention to not altering geometry just cleaning any machine marks..Correct?

[Bretandry]

I have been lapping and bedding barrels on all my builds for a while now thanks again and yes i agree it's a good practice no down side to doing it (got that practice from Joe Carlos). Just an FYI to remove a bedded barrel "Goof Off" super glue remover gets loctite to release. A decent dousing a few times before disassembly is all you need.