The great thing about the M4 Compatibility Chart (image in post 24) is that its creator Rob Sloyer took an incredible amount of time gathering data from the M4 TDP (Technical Data Package), boiling it down to 21 specs he believed mattered the most then gathered corresponding specs for 15 popular M4geries. He then made an easy to read spreadsheet ranking them according to adherence to the TDP.
The bottom specs for rear sight/carry handle and warranty don't directly impact the quality of the rifle, more the value. And the rating for double shielded handguards doesn't matter much unless you keep the originals.
It is important to read and ultimately understand the explanations of TDP specs in the second image in post 24. This tends to get separated from the chart but shouldn't.
Another thing to remember, this chart ranks M4geries not varmint, target or specialty versions (where TDP specs may or may not be advantageous). Technically it doesn't even rank midlength gas, only carbine length gas normally on a 16" barrel. However it's reasonable to expect a slightly different barrel profile that otherwise has the same specs will be of similar quality.
The unfortunate thing about the chart is that it no longer is maintained (no doubt too much work) so you don't see ARs that have become popular over the last few years. However if you understand how the chart works then get the specs of PSA, Spikes, Windham and others it's obvious where they would fit in. PSA would drop in right around Noveske while Windham would drop in around Bushmaster since it's built like the old BM. Spikes, I don't know for sure but should do well.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek
Last edited by Quentin; 12-15-2012 at 07:24 PM.