Anyone have good sucesss running 77 SMK's in an AR ?

To make up in speed for what the OP misses in twist requires an unacheivable increase in muzzle velocity.

If the 77SMK is nominally moving out at 2,750fps-or-less (which is the measured velocity of BlackHills 77SMK ammunition in my 1:7.7 20" barrel), I would need to increase that to 3,220fps to accommodate a 9-twist.**

'Just ain't gonna happen, especially with shorter AR barrels



**(Note: That same 20" barrel twist in an earlier 1:9 would not stabilize the 77SMK)
 
Instead of the 77 SMK, try dropping down to a 75gr Hornady BTHP. Not the Amax, just the regular 75gr Hornady.

It is slightly shorter, you can launch it slightly faster, and stability is slightly better from a 1:9 twist. Might be what you need to get a long range load for your rifle.

Jimro
 
77 SMKs
23g w748
2.26 "

20" PSA CHF barrel--- 1:7

I just picked up some TMKs, havent had a chance to
try them out yet.
 

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More to the OP's point, the "AR" per se, can handle 77SMKs just fine -- with the proper twist:
10h0fvo.jpg
twist:
 
The OP stated he is using a 24" barrel . If the blackhills is getting 2750 from a 20" barrel . The OP should get at least 2880fps from his 24" barrel . That's why I used 2850fps as my base velocity . A 1-9 twist will stabilize a 77gr smk , just not well . ( so who's going to be the one that says a bullet is either stable or it is not ? )

I think uncle nick had it right :)

The 75gr Hornady is an option but my guess is that it will not do much better . How ever the Berger 73 gr Match BT Target bullet has a recommended twist of 1-9 . They cost a bit more but if the 69gr smk is not good enough I'd try the 73gr Berger http://buybergerbullets.3dcartstores.com/22-Cal-73-Grain-Match-BT-Target_p_31.html.
 
Metal God,

Let me put some math to my previous post.

69 SMK's are nominally 0.900" and the 75 Hornady 0.981, and the 77 SMKs 0.994, the 73 Bergers (match target) are a 0.970 length. In inches.

Stability factor for a 1:9 barrel shooting the 69SMK at 2,800 is 1.656
Stability factor for a 1:9 barrel shooting the 73BGR at 2,850 is 1.420
Stability factor for a 1:9 barrel shooting the 75HDY at 2,850 is 1.412
Stability factor for a 1:9 barrel shooting the 77SMK at 2,750 is 1.378

Since we know that the OP is looking at marginal stability at longer ranges we won't know if the increase in stability to the 75HDY from the 77SMK is enough to get him to the 600 yard line for competition. My recommendation is to try, after all a box of 75gr HPBTs is relatively cheap.

69SMK 0.169 G7 BC
73BGR 0.176 G7 BC
75HDY 0.183 G7 BC
77SMK 0.190 G7 BC

The one other factor that gives me some hope for the 75HDY is that it is a secant profile bullet which maintains flight stability better than the tangent ogive bullets (like either SMK), at least until you hit the transonic region, so it is more likely that the OP will get longer stable flight.

But, if the 75HDYs still provide no performance advantage, the 73 Bergers would be a logical next solution.

Jimro
 
77g SMK

"so who's going to be the one that says a bullet is either stable or it is not?"

The bullet at/on the target...:rolleyes:

You are not going to beat the laws of physics no matter how much you kick the dead dog...

T.
 
So here's where I'm at. I want to try one more powder which is my fav ball powder good ol' Win 748 . Hopefully get out tomorrow and see what happens at 200 meters. If I don't find anything around 2 MOA or better (which is the best I've gotten so far using Benchmark powder) I'll go ahead and load up my Benchmark recipe and shoot them at 600 yds next Fri . I'll put the "stable or not " speculation to bed. Stay tuned for an update..
 
Got out the the range yesterday and to my surprise I had excellent results with Win 748 powder and the 77's. While I didn't observe much better accuracy than Benchmark powder I did see some data that suggested more consistent accuracy. Found a good node at both 23.4gr and 24gr. The velocity averages were 2656 and 2753 respectively. Slightly better overall accuracy average with the 24gr charge. Excellent stability at 200 meters. So i'll think I load up the 24gr/W748 load and see what happens at 600 yds.
 
Pleast post your results. The ballistic calculators say you should be good, but reality trumps all.

Jimro
 
Your rifles 1n9 twist doesn't like the 77gr bullet.... If you ever consider a new barrel get at least a 1n8 or faster.

I also suggest you try some 75gr Hornady.
 
A stability question to the mid range experts , in my situation experimenting with the less than ideal 77gr's will stronger winds elevate the possibility of bullet instability ? My 600 yd range is notorious for being a challenging multi-directional wind tunnel !
 
Short answer yes . uncle touched on it a little . I'll use an example . If you have two tops . One spinning very fast the other spinning just enough to stay upright . The top that is spinning very fast will be much harder to destabilize then the slower one . Even though they are both stable while spinning .
 
A stability question to the mid range experts , in my situation experimenting with the less than ideal 77gr's will stronger winds elevate the possibility of bullet instability ? My 600 yd range is notorious for being a challenging multi-directional wind tunnel !
Click to expand...

Yes, and rapid direction changes will increase bullet yaw speeding the destabilization effect.

However, if you can't get the 77 SMKs to work for you, and the 75 Hornadys aren't any better, try working up to max loads with 68 Hornadys or the 69 SMKs.

A 68 Hornady at 3000 fps has less drop than a 77 SMK at 2800 fps, and only 0.3 MOA difference more windage out to 600. The ballistic data for the 69 SMK is essentially the same for drop and windage.

If you can get the 75 Hornadys to 2900, you can match the 77SMK wind corrections when the 77 SMK is launched at 2800.

The go to powders for all the loads listed are H4895 or Varget. Both are "temp stable" and can on paper achieve the velocities listed for the 68 through 75 gr bullets.

Hope this is helpful.

Jimro
 
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