I have seen this question asked and "answered" over and over. I decided to answer it for myself.

Materials:

Colt HBAR 6601 AR-15A2
PMC 55gr .223 ammunition
M855 Winchester ammunition
Hornady 40gr VMAX ammuition (advertised at 3800fps)
3/8" thick mild steel plate
Spray paint
Pine 2x4's that have sat outside for approx. 2 months.

I have heard it said that the velocity of M193 or .223 55gr will allow it to out penetrate M855 within 25-100yards in steel. After that, the M855's steel core starts to matter.

So I put that "conventional wisdom" (Velocity is what penetrates) to the test. Here are my test results:
i18.tinypic.com/72r9l4h.jpg
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i14.tinypic.com/6wzr8fn.jpg
i12.tinypic.com/8e8xh5w.jpg

PS. Shooting offhand with A2 sights at 140 yards at dusk was a challenge, but I managed to tag the plate twice (out of something like 6 or 7 shots [B)])

This being said I could not get ahold of XM193 for the test, however PMC .223 is VERY close as you can see from these chronograph tests.
PMC .223 Chronograph data
M193 Chrono data (LC)
If the M855 does it at 140 yards and the PMC wont at 25, I feel that proves the point even IF one wants to argue the minute velocity difference with XM193 vs. the PMC I used.


tried some Q3131 and it DID suprise me, results can be found here: http://ar15.com/forums/topic.html?b=3&f=16&t=351903