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Old February 11, 2013, 09:25 AM   #1
Skans
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AR Receivers - smooth vs. not-so-smooth texture???

I just purchased a set of SI Defense billet upper/lower receivers. As I'm assembling the lower receiver, I have an older Colt SP1 Carbine next to me so that I can remember how to fully assemble the receiver set. (putting it together was no problem).

Aside from the receivers looking slightly different than the old A1 style, the biggest thing I noticed was the finish. The finish on the SI Defense receivers is hard-coat anodized. I don't know what type of anodizing process they used on the old A1 style receivers. But, the older Colt receivers are very slick to the touch, especially inside the upper receiver where the BCA goes. As I ran my fingers inside of the new receiver set, it wasn't exactly rough, but it wasn't slick like the AR receivers.

Has anyone noticed this with their AR builds? Has anything been reviewed about the "smoothness" of the upper receivers? In a side-by-side comparison, this really stood out - just wondering what others think about this or have experienced. Does this matter at all?
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Old February 11, 2013, 10:30 AM   #2
Quentin2
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It's really just a cosmetic issue. I noticed in my last build that at first it felt rough racking back the charging handle. No functioning problems though and after the first 100 rounds it smoothed out fine. I'd say the BCG will quickly smooth out the receiver's normal surface roughness in mating areas. Lube it and shoot!
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Old February 11, 2013, 03:29 PM   #3
blfuller
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It all depends on the grit used when the abrasive blasted the parts. The Colt are all hard anodized as well. The interior of the Colt upper had a dry lube applied to the inner diameter which is why it is slick. Since the Colt is made from a forging and the SI was machined from bar stock the SI will probably be rougher. The forgings are pretty smooth and to remove any machine marks they probably had to use abrasive blasting on the SI receivers.

The thickness of the hard anodizing also will determine how smooth it is. Hard anodizing builds up on the surface and also penetrates into the base metal. Another factor is the alloy used. Colt Mil spec typically used 7075 T6 forgings.

As far as it mattering, probably doesn't as long as they did per the specification. The thicker hard coat will probably wear better. Some are dyed black after anodizing and some are have a painted on finish. The early Colts are grayish which may be the natural hard coat. Age may have a factor as well depending how many times it has been cleaned, etc.
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Old February 11, 2013, 06:35 PM   #4
Skans
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My older Colt Sp1 Carbine has been shot very little, and the A2 forged upper receiver hasn't been shot at all - both are forged, older and very smooth to the touch on the inside. Perhaps this doesn't matter, and maybe after using some lube and shooting the newer receivers it will smooth out some. I just found it odd that there would be that much difference in the texture inside the receivers.
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Old February 11, 2013, 08:48 PM   #5
blfuller
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As I mentioned, the Colts had a dry film lubricant that was applied to the inside of the upper receivers. This was sprayed on like a paint and allowed to cure. That is why they are probably so smooth.

I looked at the SI defense website and they just callout that their receivers are black anodized. That would tell me that they are not hard anodized but a Type I or II, class 2 . If you really want to find out, call them and ask them what specification their finish meets. Mil-A-8625 Type III is hard anodizing.

Again you are not comparing apples to apples here. The Colt receivers are milled from a forging and the SI are machined from billets, the Colts are hard anodized and the SI's are probably just type II anodized and the Colt receivers have the dry film lubricant applied to the inside of the upper.
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