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The reliable frag threshold for the 77gr SMK is 2300 fps

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OP,
   Jeff Hoffman & Carl from BHA have always given us nothing but highly credible, verified information on the questions we asked about their ammunition. BHA tested the 77Gr. TMK down to 1900 FPS and it still expanded. IIRC, Jeff stated that the 77GR. TMK might expand at a slightly lower impact velocity but BHA had not tested it below 1900 FPS. The 77GR. TMK & the 77GR. SMK bullets have some major differences. The TMK is longer than a similar weight SMK. The TMK has a very large hollow point under the resin tip which allows hydraulic expansion. Conversely, the SMK has a very small open tip that will NOT allow hydraulic expansion. Because of this, JAG would almost certainly not approve the 77GR. TMK as a Hague compliant bullet design. The TMK is not a yaw dependent bullet. Conversely again, the SMK is a yaw dependent bullet. Sierra emailed me that the 77GR. TMK does not even use the same jacket as the 77GR. SMK. Sierra's calling the TMK a Matchking bullet was a misnomer. Best to think of the TMK & SMK as different bullet designs. As far as application is an SBR: The 5.56MM 77GR. SMK will fragment out to approx. 75yds. from my 10" M4 where as the 5.56MM 77GR. TMK will expand/fragment at better than 2X that range from the same 10" M4 under the same conditions. HTH

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BHA manufactures their 5.56MM 77GR. TMKs at a mag-length COAL (Cartridge Over All Length) that yields 100% reliable feeding from any high quality mag such as a mil-spec mag or PMAG. The longer 77Gr. TMK bullet is seated deeper into the cartridge case than the 77GR. SMK. So the BHA 5.56MM 77GR. TMK and the BHA 5.56MM 77GR. SMK share a similar COAL. .02

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This picture is several years old: Exit wound on a 180LB. razorback from a BHA 5.56MM 77GR. TMK fired from a suppressed 10" M4@50yds. IMO, a single 77GR. TMK to the center of the chest at SD/HD range is not survivable short of transplanting organs. But a DT is good practice for proper muscle memory. This razorback hit the ground and never got back on his feet, dead in seconds. .02

EDIT: The below pic is several years old now. I have taken dozens of game-shots with the 5.56MM 77GR. TMK since then all with the same high level of terminal performance. But the bonded/homogenous bullet types are better suited to deep and/or barrier penetration. I went thru Hurricane Harvey & its aftermath with a mag of BHA 5.56MM 77GR. TMK in the carbine with a spare mag of 5.56MM 70GR. TSX, my preferred 5.56MM hunting ammunition types. I seal all the hunting cartridges for the rifles & the HST cartridges carried in the Glock with lacquer just as a precaution.  But I never dreamed that I would be needing sealed ammunition in Kingwood, TX. Live & Learn. .02

http://preview.ibb.co/b2D4g5/5_56_MM_77_GR_TMK_from_10_M4_50yds.jpg

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I don't doubt their credibility. I didn't hear it from them though, I read the 1900 fps threshold on here, and the only test I have seen with their 77 grain ammo close to that threshold was with the smk version and it didn't perform at that velocity... I figured the two would work differently, but 400+ fps is a pretty big difference.

So before I spent about $60 on the BH TMK loads I wanted to verify what I had read about the TMK threshold(which will work) vs the closest test I have seen with the SMK (which wont work)

Being longer, does that effect how they feed from P Mags at all?

Thanks for your help..