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Good SD but bad groups?

C

Corse

Private
Minuteman
Jun 26, 2018
33
7
I have been trying to get some Nosler 70gr RDFs to shoot in my AR. I tried some 748 today with both the RDF and 69 TMK.
The RDF had some decent SDs (8.8) but absolutely horrible groups, I didn’t measure, but 5” would be generous. The TMK had a SD of 1 for two separate charge weights and the third was 0, but similar groups to the RDF.
I shot a control group with 52gr MK and it was typical of the gun @ 0.4”.

Has anyone experienced anything like this before?
 
Corse said:
I have been trying to get some Nosler 70gr RDFs to shoot in my AR. I tried some 748 today with both the RDF and 69 TMK.
The RDF had some decent SDs (8.8) but absolutely horrible groups, I didn’t measure, but 5” would be generous. The TMK had a SD of 1 for two separate charge weights and the third was 0, but similar groups to the RDF.
I shot a control group with 52gr MK and it was typical of the gun @ 0.4”.

Has anyone experienced anything like this before?
Click to expand...

SD's tell you how well you're ammo is performing (its consistency); it doesn't tell you how well the rifle is performing with that ammo or how well the shooter is performing. For example, you can have SD's like stated, but the load can be outside of an accuracy node. So, in terms of shooting for precision/accuracy it's better to pay attention what what you see on paper than SD's.
 
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spife7980 said:
Yeah, quit chasing meaningless numbers and see what the target says.

If anything is shooting 5" at 100 yards then you have something else wrong besides the bullet itself.
My bet is that you have a slow 12 twist barrel if the light bullet does well.
Click to expand...
No, it’s a 1/8. The 69 and 77gr both shoot well with other powders.
 
I haven’t tried it with the 77s.
I heard good things about the 69 MK WITH 748, so I thought I would give it a try with the RDF and TMK.
 
As others have said, sd/es has zero to do with group size at 100. SD/ES is a product of your brass prep and powder charge weights consistentcy.

Try 8208 or other powder, once it starts shooting better adjust seating depth to dial group in.
 
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Doom said:
I’m curious, you had a standard deviation of 0, and 1? How many shots were in each of those SD’s?
Click to expand...

They were only 3 shot groups. I loaded them to test 748. I usually use XBR or H322. Supply is inconsistent, especially with XBR, so I was trying to find alternatives.

i was surprised by the great SDs and horrible accuracy, never seen anything like it before.
 
Corse said:
They were only 3 shot groups. I loaded them to test 748. I usually use XBR or H322. Supply is inconsistent, especially with XBR, so I was trying to find alternatives.

i was surprised by the great SDs and horrible accuracy, never seen anything like it before.
Click to expand...

For ARs I do 10-shot groups to t/e different loads. 3 shots won’t tell you anything and taking data on such a small sample size is counter productive. Besides, groups size at 100 is as much about the shooter and system as it is the ammo itself. I’d try some other combos and do 10 shot groups. SDs in the teens or even low-mid 20s are more the norm for the typical AR rifle/load. My Block II (14.5” 1/7) usually yields mid-teen SDs for black hills COTS 77g OTM; I still can regularly get hits at 600 on a 12” plate in decent conditions when I do my part.

Try TAC if you haven’t already.
 
Last edited:
Put the chronograph away, or ignore SD on a three shot group. Mathematically, it calculates but the sample size is meaningless.

I understand the XBR pain. It’s all I use in 223 after learning about temperature sensitivity of H335 when I first started reloading. I hate dealing with the ADI powder availability, i also have a very good load for it in 308.
 
OMG! You're saying you had 5"+ Three-Shot PATTERNS? WOW!!!
So, IMHO, the AR is its own dynamic. You are dealing not only with the barrel factors same as with any bolt gun but the gas system's ability to move the bolt and carrier in a consistent motion. I agree that the chronograph should be the last order of business. Get acceptable accuracy and then find out about speed. Don't worry about tuning the SD (it never killed anything) but watch ES instead. If you plan on trying targets at distances past 600 yards or so low ES will lessen the opportunity for vertical stringing.
 
Along with the above mentioned powder changes, like Ramshot TAC, I would try 4895. Also what is your bullet seating depth? My understanding is the RDF’s don’t like being jammed into the lands; I would start .050” off the lands. A 1 in 8 twist should be fine as long as the crown is good and barrel is in good shape.
 
If your AR has a mil spec chamber then I’m betting you have quite a jump to the lands. The RDF is going to be more sensitive to jump than SMK. Don’t know if that’s what is going on here or not.
 
It is a Wylde chamber. I seated them to just under max mag length (2.255”). Normally I would do more testing and shoot a larger sample. I was trying to save time and bullets by chronographing everything As those were the last of my TMKs.

I wasn’t necessarily chasing any numbers, I just threw the Ammo together based on what was supposed To be a proven load. Obviously something is up.

Honestly, I’m not surprised the RDFs didn’t shoot well, but I wouldn’t expect that kind of accuracy. I was just amazed at the consistency, even though it was only 3 rds each.
 
Not sure of the overall lengths but in my bolt wylde chamber using my Hornady comparator I have the ogive measurements, but in my testing .020-.030 off the lands were 👌
64F732A5-069F-4835-8726-43CE47D14DC6.jpeg


That’s going to be way long for an ar mag.
 
spife7980 said:
Not sure of the overall lengths but in my bolt wylde chamber using my Hornady comparator I have the ogive measurements, but in my testing .020-.030 off the lands were 👌
View attachment 7318231

That’s going to be way long for an ar mag.
Click to expand...
I have had decent results with XBR also, in the same range. Of course, loaded to mag length, but my velocities were lower (~2790) out of an 18” barrel with 23.8 gr. Groups in the .75“ range @100 yrds.
 
Unless you are shooting 1000yds. I would just pick the best group with lowest sd and es.
 
Did you clean between component changes? I've found that it can matter to accuracy. Usually not 5 moa for me lol, but it will definitely have considerable dispersion
 
Ryridesmotox said:
Did you clean between component changes? I've found that it can matter to accuracy. Usually not 5 moa for me lol, but it will definitely have considerable dispersion
Click to expand...
I did not. I did go out and try again with just the RDFs. Results were slightly better at lower charge weights, but I am not perusing this combo any further.
 
I don't believe your are getting SD of 0 and 1 cfs in a 223/5.56. I'm happy when get a st dev in the teens with that cartridge. But I can consistently put them under 1/2". Zero and 1 are simply not believable. If you're skilled enough to get even high single digits with a 223/5.56 cartridge, then I would think you'd have no trouble figuring out how to get sub MOA accuracy. Not trying to be a dick, just say...
 
I just saw where you were only using 3 shots to calculate. That's not going to give you much to go on. Even with just 3 shots, it still seems unlikely to get 0s and 1s. Maybe just a freak occurrence. Or your chrono is stuck.
 
Jayhawkhuntclub said:
I don't believe your are getting SD of 0 and 1 cfs in a 223/5.56. I'm happy when get a st dev in the teens with that cartridge. But I can consistently put them under 1/2". Zero and 1 are simply not believable. If you're skilled enough to get even high single digits with a 223/5.56 cartridge, then I would think you'd have no trouble figuring out how to get sub MOA accuracy. Not trying to be a dick, just say...
Click to expand...

You can think whatever you would like, but that is what the magnetospeed said. Is it repeatable, I have no idea. Also it was only 3 shot groups, so I’m sure it wouldn’t remain that low over a longer string.
 
Corse said:
You can think whatever you would like, but that is what the magnetospeed said. Is it repeatable, I have no idea. Also it was only 3 shot groups, so I’m sure it wouldn’t remain that low over a longer string.
Click to expand...

I agree.

Unless it's repeatable, it really doesn't mean anything. :eek: ;)
 
Corse said:
but that is what the magnetospeed said
Click to expand...

Yep, I have limited experience with mine but yeah, statistically speaking I saw the same. Shot a group of five and see single digit SD, then shot a string of 14 and had very different results. Kind of shocked to hear about TMKs doing so badly though. Someone, somewhere might be able to get the 70 RDF to shoot out of an AR but I sure don't know them.
 
Mike_in_FL said:
Yep, I have limited experience with mine but yeah, statistically speaking I saw the same. Shot a group of five and see single digit SD, then shot a string of 14 and had very different results. Kind of shocked to hear about TMKs doing so badly though. Someone, somewhere might be able to get the 70 RDF to shoot out of an AR but I sure don't know them.
Click to expand...
I guess I can save those RDFs for a bolt gun, if I ever get one in 223.
 
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