Sierra 77 and 80 seating depth

[urbanrifleman]

I have never really done a full seating depth workup for either.

I always shot the 77 at mag length. Does the 77 improve closer to the lands?

Does the 80 shoot better jammed?

 

[watercam]

I have always jumped conventional, non-VLD bullets .040" with good success. That said the 80s will generally need to be single loaded. I shot a Long Range match at the Marine Corps Base at 29 Palms and their rifle team used 77 SMKs in their Service Rifles to 1K.

 

[6BRAIman]

I shoot Sierra 77gr TMK in a 22BR and it definitely makes a difference in it. When I got it on up close to he lands the group’s improved.

 

[Ned Ludd]

That will depend on the specific bullet. Tangent ogive bullets such as the 77 and 80 SMKs will often tune in between about .010" and .030" off the lands, most typically at around .020" off, or thereabouts. In my hands, secant ogive or hybrid ogive bullets can be a little more finicky, but often still seem to like being seated somewhere in the neighborhood of .009" to .015" off the lands. Along that line, although I have not personally tried them, I have seen numerous posts suggesting Sierra's TMK bullets are more finicky with regard to seating depth than the standard SMKs. Unfortunately, there is really no substitute for testing seating depth in your specific setup with a given bullet so that you know for sure.

If a specific COAL (and therefore relationship of the bullet ogive to the lands) cannot be achieved due to the limitation of loading to mag length, it is still possible to optimize seating depth. Simply select the longest possible COAL that will reliably feed from the magazine, then test seating depth only in the direction of farther off the lands. It is often possible to find an acceptable seating depth using this approach, but if the bullet is moved sufficiently far away from the lands, the reduction in effective case volume may necessitate decreasing the charge weight slightly.

 

[RegionRat]

There is good enough... and then there is better... in between those two there is usually a price to be paid.

I wouldn't hesitate to test, but as often as not, the jump works out better for volume games where you must load hundreds of rounds for matches that run for several days and far from home. In those games, it helps with logistics if you find the wider jump node than to try and chase throat wear.

So, yes there have been some awesome results with the 80 SMK being jammed, but in the grand scheme of the folks who burn them by the hundreds, the jump nodes are durable and jam needs attention far more often. Again, that is mostly driven by Highpower and Service Rifle perspectives who are happy at 1/2 MOA for three or four days worth of matches at a time between tunes.

BTW, I'm sure you know but it is worth mention, that the 77 TMK sits between the 77 SMK and the 80 SMK in terms of BC. The other advantage to the TMK is the terminal performance comes closer to a hollow point than the SMK so it can be useful for distance shots on pests.

 

[hikfromstik]

I bought a Tikka t3x varmint 223 1-8" and was shocked after buying it that the freebore was so short. At 2.238" the 77gr smk's are touching the lands. They shot horrible in the lands. I backed them to 2.225" and it helped some, but still about 1 moa at 100yds. 53gr Vmax and 69gr shoot bug holes backed out .020" .

 

[LVLAaron]

If you're talking about an AR, I'd stick to mag length on the 77's.

80's in my experience like to be somewhere between touching and 20 thou off (at least in a wydle chamber)

Lots of good information on the 80's here: http://radomski.us/njhp/