140 SMK vs. 142 SMK in a 6.5 CM

[EdHarley]

I am going to switch from the 140 to the 142 in a 6.5 CM gas gun. Has anyone done this and seen an appreciable difference in the powder charge weight? I am willing to redo the load development but hope to minimize some of the work if possible.

 

[mac86951]

I am going to switch from the 140 to the 142 in a 6.5 CM gas gun. Has anyone done this and seen an appreciable difference in the powder charge weight? I am willing to redo the load development but hope to minimize some of the work if possible.

Yup, it's a change in powder charge. I'm about 0.2gr lower. Try the Lapua 139 too. I found the best results with it.

-Mac

 

[EdHarley]

That is not bad. I just don't want to go all the way back to a minimum load and work up. Didn't know if there is a significant bearing surface difference or anything else that would require this. I may go down a gn and workup by 0.3gn.

 

[Hohn]

Shorter bearing for longer 142. Pic from rifle shooter.com

 

[EdHarley]

I noticed the longer bullet and the tips are pointed too. Will that translate into less pressure, more velocity? The Sierra data is identical so it seems not.

 

[Hohn]

I wouldn’t read anything into the “identical “ data other than Hornady and Sierra didn’t want to spend the effort to optimize data for each bullet.

Id be very surprised if the two bullets gave identical pressure points or had the same nodes.

 

[EdHarley]

Makes sense.

 

[mac86951]

So for my gas gun, I found the additional seating depth the problem. I suggest the Lapua 139 since with mag length constraints, I'm able to get an increase to a solid accuracy node. BC difference between the 139 and 142 at 600y didn't make up for it. I was limited with my gas system using a 142 to ~40.5gr of h4350.

-Mac

 

[ZenArchery]

As a hunter if bullets fall within 3-5gr +\- I use the same load as long as the bullet profile is similar. Almost all have landed 1MOA.

 

[EdHarley]

I have had a heck of a time with this gun trying to get a load I like. I have one that is ok with the Berger 130 OTM and varget but would like something in the 140 range. I will shoot this box of 500 or part of it and if it sucks ( the round/gun combo, not the bullet) get the Lapua.

 

[mac86951]

You're using h4350? Try the 142 a little slower, say 40.0gr and work around that. Fed 210M primers, or cci-BR2.

 

[EdHarley]

I have it but Varget and RL 16 shot better. I can go back to it and see if this bullet is better tolerated.

 

[watercam]

BC is higher for the 142 as well...

 

[STOMP442]

The 142 is an excellent bullet. Works great on game too if you were ever to use it for that.

 

[EdHarley]

That is good news. So by slower, Mac, do you mean in reference to the speed I was shooting the 140 or is there a specific velocity you think works? My barrel is only 20" so I was only able to get in the 2570fps range with them at 40.2gn. With RL 16 I was getting 2700 at 41.8gn.

 

[mac86951]

That is good news. So by slower, Mac, do you mean in reference to the speed I was shooting the 140 or is there a specific velocity you think works? My barrel is only 20" so I was only able to get in the 2570fps range with them at 40.2gn. With RL 16 I was getting 2700 at 41.8gn.

In reference to the 140s. My 24" barrel the 140s we're in the 2650 range, the 142s preferred the 2600. I had a heck of a time getting seating depth right to mag feed; by the time I got it, COL was nearer to 2.775" and the 50fps download for the 142 would start showing pressure problems on a hot day. That convinced me that the 142 for an AR is really a bit excessive, going back to the 140/139s and even lighter. If all you're doing is punching paper; a real world test of 123s is worth while if you're shooting within ~600y.

I'm currently running 139s with 41.0gr h4350 which is just shy of 2700 from a 24" barrel. Yes the 142 has a better BC, because that sounds cooler at the bar; or the internet. In my rifle that calculates out to a reduced wind drift at 1000Y. Awesome! Is it worth the extra time? Well, that reduced wind drift at a full value 10mph (1000y) is about 1/3 of a minute. Or essentially 1 click and a smidge of holdover different on a 1/4moa per click scope. If I'm shooting solid 10s and drop a 9 in F-class; that bullet may be the difference; but I'm not there yet.

I'm curious to see how you do with 142s, so take my experience with a grain of salt (my load, my rifle) and keep us posted.

-Mac

 

[Rob01]

I have had a heck of a time with this gun trying to get a load I like. I have one that is ok with the Berger 130 OTM and varget but would like something in the 140 range. I will shoot this box of 500 or part of it and if it sucks ( the round/gun combo, not the bullet) get the Lapua.

What other bullets have you tried? The 140 SMK is a dog in 6.5 bullets. Terrible BC for the weight. Have you tried the 140 ELD-Ms?

 

[EdHarley]

What other bullets have you tried? The 140 SMK is a dog in 6.5 bullets. Terrible BC for the weight. Have you tried the 140 ELD-Ms?

I have tried the 123 ELDM, the velocity/SD/ES were ok but the groups sucked. I have also used the 130 OTM by Berger. Decent load but was looking for a 140gn bullet. The 140 was shorter and I predicted would be easier to shoot well within the constraints of the magazine length bullet but it turned out not so much. I am going to sort the 142s this weekend and shoot next week. Plan to use the same base to Ogive length minus whatever I have to reduce to fit in the magazine. I have H4350, RL16 and 4831sc to use for powders.

 

[Rob01]

You shouldn’t have an issue with 140s at mag length. 147s are loaded to mag length also. Try the 140 ELDs. They have excellent BCs.

 

[EdHarley]

You shouldn’t have an issue with 140s at mag length. 147s are loaded to mag length also. Try the 140 ELDs. They have excellent BCs.

Good to know. I shot the 147s in a box of factory ammo and they did surprisingly well.