Seating depth of 77 gr MatchKing

[cpatbay]

I know the absolute max OAL for .223 Remington is 2.260" and the "correct" way to determine the preferred OAL for my rifle is to measure with an Overall Length Gage (like the Hornady one). I want to experiment with a few shorter than 2.26" OAL but I am wary of higher chamber pressure at shorter OAL with these long 77 gr bullets.

So, what would be a reasonably "safe" min OAL I could use as an ending point?

[rg1]

I seat them to 2.250" and they show no pressure signs with book loads. I find them to feed more reliably in my more than one AR. I can't see any accuracy difference seating them .010" shorter than magazine maximum length.

[brando]

Yeah, what he said. They're pretty long bullets, so you're not going to be able to seat that much deeper than mag length, but there is about 0.01" to play with. Personally, my Mk262 clone load is right at mag length.

[Jonathan Doe]

Sierra 77 grain MK is designed to be seated to magazine length. I load to the maximum length that would fit in my magazines.

[Ahhnother8]

I want to experiment with a few shorter than 2.26" OAL but I am wary of higher chamber pressure at shorter OAL with these long 77 gr bullets.

The deeper you seat them the lower the velocity, caused by lower chamber pressure. Shoot over a chrono to see for yourself.

[cpatbay]

The deeper you seat them the lower the velocity, caused by lower chamber pressure. Shoot over a chrono to see for yourself.

This is counter intuitive, isn't it? If you seat it deeper, the volume in the case reduces. As powder burns, lower volume should result in higher pressure, right?

[rumblebee]

I just got back from the range and was testing the 77gr SMK's. My COL: 2.243 with 23.5gr of Varget. Got a .600 (5) shot group on a pretty windy day. I also tested 2.260 and was not happy with it. My Tikka really seems to like COL:2.243 with the 77gr SMK's and the 75gr Horn BTHP.

Good luck

[popeye4]

This is counter intuitive, isn't it? If you seat it deeper, the volume in the case reduces. As powder burns, lower volume should result in higher pressure, right?

The bullet moves, increasing "combustion chamber" volume rather quickly. You get into pressure issues when you seat the bullet into the lands, significantly increasing the resistance to initial bullet movement. I don't think seating deeper into the case is going to significantly change the pressure, unless you are getting really silly about it. I also don't think you'll see much difference in accuracy, as the Sierra 77 gr MK is designed to "jump" (for the 300 rapid fire event in highpower, you HAVE to load from the magazine).

[popeye4]

I just got back from the range and was testing the 77gr SMK's. My COL: 2.243 with 23.5gr of Varget. Got a .600 (5) shot group on a pretty windy day. I also tested 2.260 and was not happy with it. My Tikka really seems to like COL:2.243 with the 77gr SMK's and the 75gr Horn BTHP.

Good luck

Keep in mind that the true datum point you should be measuring from is the diameter where the bullet contacts the rifling, not the tip. There are tools made specifically for making this measurement. Base to tip dimensions can have quite a bit of variability, particularly in HPBT match bullets.

[cpatbay]

The bullet moves, increasing "combustion chamber" volume rather quickly. You get into pressure issues when you seat the bullet into the lands, significantly increasing the resistance to initial bullet movement. I don't think seating deeper into the case is going to significantly change the pressure, unless you are getting really silly about it. I also don't think you'll see much difference in accuracy, as the Sierra 77 gr MK is designed to "jump" (for the 300 rapid fire event in highpower, you HAVE to load from the magazine).

So, I should seat these 77 gr MK deeper for optimum performance? What would be considered "silly" deep?

[popeye4]

So, I should seat these 77 gr MK deeper for optimum performance? What would be considered "silly" deep?

No, seat them to magazine length. They are designed to be relatively insensitive to "jump", so varying the "jump" shouldn't affect it much. This was out of necessity. These bullets were designed to be used in the rapid fire stages of highpower matches in AR15s (200 yd and 300 yd). They replaced the 69 gr. SMK. For the 600 yd stage, the 80 gr. SMK is used, but it won't fit a magazine, so the 77s are a compromise design. In order to fit the mag, they HAVE to jump. But they aren't better than the 80s (or 90s). Typically, though, the less jump the better the accuracy. How many shots are you using to conclude there is an effect? A 3 or 5 shot group is not going to be statistically meaningful.

Have you measured your rifle's chamber to determine exactly where the bullet touches the rifling? This is a good dimension to know if you are varying bullet seating depth.

"Silly" deep would be a judgement call, but pushing them back way into the case (much farther than magazine length) would qualify; I was imagining just the tip protruding from the case. You might start compressing the load.

[rumblebee]

10-4 on the below. I made my own tool, which is an applicable case (.223 or .308 neck split with dremel) and the applicable bullet (I run COL for each bullet I am loading). I'm not sure why, but my Tikka likes the "jump" and performs as well (if not better) than the COL's that were just off its lands. I've tried many different combinations.

I then read the below and gave up on the finding my magical distance to the lands (for now)

http://optimalchargeweight.embarqspace.com/#/tools-page-3/4529825348

Anyway..not intending to hijack the OP's thread, just thought the Dan's above approach was interesting.

Keep in mind that the true datum point you should be measuring from is the diameter where the bullet contacts the rifling, not the tip. There are tools made specifically for making this measurement. Base to tip dimensions can have quite a bit of variability, particularly in HPBT match bullets.