Bolt carrier comparision, any difference?

[DM-SC]

Is this chromium one by DPMS GTG?

http://www.dpmsinc.com/store/products/?prod=2102

I can't speak to the quality of the that BCG but, for about $34 less, you can get a LMT or BCM brand BCG.

[Bill Alexander]

Bolt carriers need a little examination.

Bores, concentricity aside, need to be chromed as these are running surfaces. The bolt tail which is nominally 0.2503" diameter runs through a 0.2510" diameter ring at the back of the carrier that provides the gas seal. This is a mechanical clearance so there is naturally some gas leakage during operation. The chrome in this area must be smooth to allow the bolt tail to run as the first alignment surface and also resist gas erosion (hard chrome is between HRC68 and HRC72 if correct) The second diameter within the carrier is the area that the gas rings run on. Gas rings are 302 stainless and as such are not particulaly hard. The surface must be smooth to prevent excessive wear on the rings. This said they are a sacrificial item and replacement between 2000 and 3000 rounds should be a consideration in a working gun. New rings will show slight shear marks when installed. After break in the edges should be square. If any rounding is observed the rings should be removed and discarded. Linear striations on the rings indicated the chrome bore is not smooth enough. The third diameter in the carrier is also a bearing surface. The bolt is supported in this area by the raised ring seen about half way down the bolt body. Again a smooth hard surface is required. In examining the bores the chrome should appear uniformly light grey and have a slight sheen. Carriers with polished bores usually result from a secondary operation to polish off excessive chrome or hide a defective finish. It is also a consideration in examining the chrome that there should be a witness visible between 1/3 and 1/2 into the cam pin path. This prevents the exposure of an edge at the point where the cam path breaks through the bolt into the bore, where chipping would occur.

Carriers are produced from AISI 8620 and typically case hardened. Specification carriers should not be subjected to secondary straightening. I have no evidence of the detrimental effects of this process beyond the induced stress in the part but would aviod such carriers.

Externally the carrier base should be smooth as this is the surface that runs over the rounds presented by the magazine. A rough base will act as a linear brake on the carrier and the speed of the carrier will change as the magazine tension relaxes. This is not conducive to reliability. The back of the carrier again should be smooth and correctly chamfered on the external and internal diameters. Lastly the front rails should be well defined without blurred edges that indicate excessive blasting of the part prior to phosphate.

Someone posted that the carrier is akin to the crankshaft in an engine. This is not a bad analogy

Bill Alexander

[markm]

Wow! Excellent post. That points out a lot of areas where the shady manufacturors like OLYMPIC ARMS could cut corners.

So springing for those COLT carriers over the years was not a foolish effort.

[C4IGrant]

All the gas keys I've seen have been chrome lined, did you mean externally chrome plated?

The carrier doesn't appear to me near as important as the bolt, just a conduit for the gas. The bolt takes the majority of stresses.

The reason I'm asking is I have a stack of new DPMS carriers outfitted with new FN (actual MPF bolts) and was thinking about getting a batch NP3'd.. if the DPMS carrier is suitable.

Thoughts?

Also has anyone actually melonited a bolt group?

I think it is pretty hard to tell the difference between one carrier from the next. This is why we ONLY buy BCM, LMT or USGI contracted carriers.
What matters (to me) is that the gas key is high quality (heat treated, chrome lined) and the screws used (heat treated).

We bought some USGI bolts (in the white) and are going to get them melonited very soon.


C4

[C4IGrant]

Wow! Excellent post. That points out a lot of areas where the shady manufacturors like OLYMPIC ARMS could cut corners.

So springing for those COLT carriers over the years was not a foolish effort.

LOL, you seem shocked to find such technical information from some of the most knowledgeable gun builders in the industry.

This post is what this forum is about (not underwater bump firing).


C4

[Shihan]

Wow! Excellent post. That points out a lot of areas where the shady manufacturors like OLYMPIC ARMS could cut corners.

So springing for those COLT carriers over the years was not a foolish effort.

I had an Oly carrier with zero chrome lining what so ever.

[Boss Hogg]

I can't speak to the quality of the that BCG but, for about $34 less, you can get a LMT or BCM brand BCG.

Those aren't chromium plated, it appears.

[markm]

I had an Oly carrier with zero chrome lining what so ever.

Yep. I remember you posting that before. I'm not suprised that they would do this.

[Submariner]

So springing for those COLT carriers over the years was not a foolish effort.

Sig line material.

In the ten years prior to the AWB, your choices were Colt (hard to find and expensive) and not-so-Quality Parts (now Bushamster). There may be better choices than Colt today but it is still the gold standard.

[markm]

Used to be you could steal COLT bolt groups for a modest premium over the aftermarket poop. It's not so easy any more.