BCM KD4 upper question

[ssc]

I am looking for any information, good or bad, about this upper. I have been shooting more 77 and 75 grain bullets and have considered this upper, but there is not much in the way of independent reviews. Any information is appreciated.

Cheers, Steve

[ssc]

It would appear these must have not been very popular or they have been flying under the radar. They have a SS 1/7.7 barrel and I was curious how they shoot the heavy bullets as opposed to the standard BCM ss barrels.

Cheers, Steve

[jason el cuerdo]

This is pretty basic info that I assume you already know but seeing as how nobody else has chimed in:

I managed to land a spot in one of his carbine classes back in '18. He was running one of those uppers and talked about it a bit.

He's a big fan of mk262 and apparently (unsurprisingly) has enough pull with BCM to have barrels cut with different twist rates until he found what worked best with that specific load. Presumably it shoots other heavy loads well too.

I haven't seen much feedback about them (like you, I suspect they go under the radar) but my vague recollection is that they are solid performers.

Unfortunately the list of gear I want vastly outpaces my finances, but I'd like to pick one up someday.

[ggammell]

So the difference between this 1:7.7 and a 1:8 twist barrel is .07 more twist. How much can .07 of a rotation have on a bullet?

I will see Kyle on Monday and I’ll try to ask.

[markm]

So the difference between this 1:7.7 and a 1:8 twist barrel is .07 more twist. How much can .07 of a rotation have on a bullet?

I will see Kyle on Monday and I’ll try to ask.

Not even measurable unless there was a decent size sample of each. If you just shot a few barrels of each, you couldn't really learn much because each barrel is its own animal.

[ssc]

Jason thank you for your insight. Mark and gg, those are my thoughts as well. I look forward to any further info that gg can offer if he speaks to Kyle. I did reach out to BCM and they said ,"That twist rate was designed and spec’d by DEV Group for the 77."

Cheers, Steve

[ST911]

The BCM KD4s I've seen have been GTG. I don't have data or group pics, but they held their own with everything else on the line shooting mostly Mk262 and a few other loads.

On twist in general... The farther down the pecking order you go the less definitive your twist is going to be. Your 1/8 may be closer to 1/9 or 1/7, and so on.

[MQ105]

So the difference between this 1:7.7 and a 1:8 twist barrel is .07 more twist. How much can .07 of a rotation have on a bullet?

I will see Kyle on Monday and I’ll try to ask.

Your math is a bit off... .3 difference...

[ggammell]

Your math is a bit off... .3 difference...

If a 1:8 twists twice in 16”, a 1:7.7 twists 2.0779 times in 16”. That would be .0779 more twist in the same barrel.

Go back to school.

[MQ105]

The .0779 number represents difference in bullet revolution while in the barrel. How many revolutions the bullet makes while in the barrel isn't what we're talking about, and is not what determines stability. Rate of twist is what the barrel twist numbers represent. For a given velocity of a bullet at the muzzle, length of barrel doesn't effect rotational velocity. Hence, rate of twist. I may need to go back to school to do the math on the difference in rotational velocity between bullets at a given velocity exiting 1:7.7 vs 1:8 barrels. However, the difference in twist rate between those same two barrels is 0.3.