Confusing Results with SMK 77gr – 223

[dbletap_ed]

I normally use SMK 69gr bullets but thought I would try some SMK 77gr bullets and I am getting some confusing results. First off the rifle is a JP CTR-02 with between 600 and 700 rounds through it which loves the 69's in front of 24.4 grs of WIN 748.

Saturday the first thing I did was to chronograph 10 rounds with the 77's shooting 5 rounds at about 1 every 30 to 40 seconds. I then let the barrel cool for 5 minutes and repeated. The 10 shot group was not bad considering I was paying more attention to the chronograph process. I then walked down range (100 yards) and change the target. I again shot 5 shots at apx 1 every 30 or 40 seconds and could not gat a good group. After 3 attempts with 10 minutes between attempts I switched to my 69gr load and got a nice 0.25 4 shot group with the 5th taking it to 0.5 this time only waiting till the barrel was cool in front of the hand guard. I then changed the target again and let the barrel cool for 15 to 20 minutes and tried the 77's again. The first group was 0.6 and the second was 4 shoots at 0.3 with the 5th taking it to 0.4. Thinking the issue must be me I waited 5 minutes and the 3rd group was 1.2".

I know I was shooting faster than a real group shooter but this rifle has always done very well with the 69's with a 5 minute wait till the barrel was cool to the touch.

Now for the question, has anyone else found the 77's to be very sensitive to barrel temp?

Thanks

[Mbauer67]

For JP barrels I would reccomend sticking with the 69's, they work really well in the JP barrels.

[dauntedfuture]

There is no correlation to temperature and bullet weight. Since we don't know how many rounds you are shooting in a group, we have to assume its something more meaningful then a 3 round group. We also don't know how far away you were when you shot the .6, .4 and then 1.2 and have to assume it was at 100 yds. I suspect that you "got lucky" with the first groups, especially if they were 3 round groups and the 1.2" is a better measure of rifle/ ammo performance.

In general most rifles are going to have some kind of change in POI or accuracy as things heat up. this is why I am a little skeptical of uberlight barrels. At some point things heat up and parts tighten up, expand and or shift. Sometimes this is not an issue but with most barrels there is going to be a difference most of the time. Think shooting a 10 round group with from a cold more then shooting a 10 round group after two magazines. Heck, I had a 24" varmint gun that if the barrel was too hot the bullets would explode in the air and when cool make it to target, I have no idea why other then something expanded or shifted in the barrel to give these results.

If you are getting better results with a more rapid cadence consistently with you rifle set up I suggest you look at the parallax adjustment on your scope. If you shoot a rapid group you are more likely to keep the rifle shouldered and the same sight picture; with a slow group you are likely taking the rifle down from your shoulder and when you bring it up, you looking through a different part of the scope.

The other thing to consider is that your process is allowing the barrel to cool but at the same time you are heating your barrel up by keeping a round in the chamber. Since we don't know what powder you are using with 77's and WW748 is a little fast for 77s, we have to assume you are using a different powder and its probably temperature sensitive. That 30 -40 second "Cool down" is in reality an "ammunition warm up" and that load might just not shoot as well with that extra 50-150 FPS from the warm powder.

You would need to single load the 77s for a "cool ammunition" comparison.

I am not doubting that there is something going on, your rifle might shoot those 69's better and it could be as simple as that.

I could continue, good luck.

[dbletap_ed]

Yep I did not give the 77gr load - 24.3 TAC

All 5 shot groups

With the 69's if I shoot a group larger than 0.6 to 0.7 it is me not the rifle (100 yards). The rifle has done many sub 1" 200 yard groups again 5 shot.

"There is no correlation to temperature and bullet weight" that is what I thought but sure seemed that way with the 77's.

As suggested will mostly stick with the 69's but it is always fun to try new stuff.

Thanks for the replies.

[Alaskapopo]

For JP barrels I would reccomend sticking with the 69's, they work really well in the JP barrels.

I like JP barrels as well and I prefer 77's. But I have also had some good results with 69's as well. The reality is these two bullet weights are very similar.

pat

Edited April 18, 2015 by Alaskapopo