An odd request, I know, but I am looking to make the mildest lowest pressure 6.5CM ammo I possibly can, for a project.  I know that just decreasing the powder below listed minimum isn't a good idea.  I've done it anyway in .223 in the past, and ended up with lots of squibs and hangfires.  

But I want to get 6.5CM down to darned near 6.5 Grendel plus level power and performance.  Has anyone played with this?

You want a powder that is in the books but not really the best burn rate.   You want something inefficient.  Look at Varget and it’s starting load.


No I haven’t done this, at least on purpose and not at all with this caliber.  I shoot a couple .260 rifles and the loads are super close to 6.5 Creedmoor.


You might find a good node with any of the recommended powders very early/ toward the start load.   It is very common.  When I started out in the eighties I often stopped when I hit the first node.  My brass lasted forever! Ok a long time.

IM4895 is one that is usually recommended for downloading even below a start load.  Read up on the downloading reduced power loads on IMR 4895.  Call the tech line if you would rather go that route.

Personally I would look for one of the faster recommended powders that doesn’t deliver as much velocity as the other powder’s max loads.

Reduced loads using Trail Boss

Pressures generally 20K to 40K PSI

You can also try Reduced loads with H4895

Cast bullets and Unique.

and here we go .  123 gr bullets at Grendel speeds.  (2580 fps is nominal for 123 in grendel)


from
https://rifleshooter.com/2017/08/6-5-creedmoor-h4895-reduced-loads-123-smk-and-142-smk/

"The Load" Mod 2.
Light bullet, and 70-80% load ratio of Vihtavuori N32C or 40k PSI max.
Has worked in .223, .308, and .30-06.

I reduced the initial (primer) pressure 300 PSI to get the Optimal Barrel Time function to work.
Attached File

So the project, if anyone is curious, relates to a DT MDRx.   While the rifle is cool as can be, the accuracy leaves much to be desired. This appears to be most profound in the heavier calibers of .308 and 6.5 CM in particular.  The dispersion is very horizontal.  with lighter .223 however, accuracy is quite good, without that dispersion. The implication is that the frame or some other factor is flexing or other exerting exessive force that was not properly designed for, that is throwing the gun left/right.   Yes, this can be shooter error as well, but in this case, pretty sure that's not it.  Especially since zero can change by 6 MOA, based on what load is used, horrizontally..

At the same time, the world desparately needs a 6.5 Grendel bullpup.  Interestingly, an overt coversion of an MDRx to .223 (or 6.5 G if that were an option), results in teh same sized rifle, at the same mass.  There's no advantage actually commissioning a machine shop to make a 6.5 Grendel version of an MDRx, as it'll still be the same 8lb beast as the 6.5 CM version

But, combine both those factors, the question comes, what happens if I just download 6.5 CM to lighter speeds, as that will give me the still good 6.5 Grendel performance, and might decrease strain enough to drastically reduce the stresses enough, to stop the wild horizontal stringing the full power loads do?  Someone sort of did this with an expensive 6mm ARC conversion.  A project that doesn't excite me at all, as I'm not interested in spending a fortune to covert an AR10 into .224 valk, either.  I would have just made a 6mm CM barrel, and run everything else stock factory, and explored downloading, like I'm doing here, for the same weight rifle of the same capability, for a lot easier and cheaper, personally.

Anyway, irked by the odd horizontal behavior, I could just sell the MDRx, but the funny thing is, I really like the rifle, and I do like the ability to shoot 6.5 CM.  Which it does do, I got 16/20 hits on a small sihoette gong at 500 yards, it's not bad!  Just who knows where those other 4 went, could have just missed, could have impacted literally 2 feet away on a perfect hold - as that's the MDRx in 6.5 CM.  So, I am looking for a load that doesn't throw 3 MOA + flyers, when the barrel/frame/whatever says it can't take it and flex's.  The gas system is 6 position, so I'm pretty sure I can open it up enough to run on something very mild even.

So, this project.  here's what I'm going to try in large primer 6.5 CM brass:
129 gr SST, crimped (as heavy as I dare)
H 4895
29.5 gr
30.3
31
31.8
32.7

And see how she runs.

OP:
I'm curious to understand what you are trying to accomplish here.....  

But I don't own one of these rifles.......

Do I understand your concern here correctly?  

That you can switch barrels (calibers) easily on this thing?  
But when you do, you see a shift of zero?  
So, say, the 223 zero is here and then you put on the 6.5 barrel the zero is in a different spot?  

And your solution to this is to modify (down load) the ammo?

Or that the 223 barrel doesn't have a horizontal stringing issue but the 6.5 does......... ?????

I think you are chasing the wrong ghost. What does DT say? Do you have one of the amazon borescopes?

Results were fairly meh,  I'll analyze more closely tonight, but no home-runs.  Strangly, I never had to open up the gas system any, even at 2200 fps loads, which seemed odd.

As to other inputs, heh, the MDRx is a whole other animal, and its accuracy issues in the medium size round catagory is repeatedly being seen form among those who bother to actually do more than youtube "it works!" Or cherry pick one good group.   It's a whole different animal with all kinds of different nuance that neither my AUG (223 bullpup), nor my AR10 in 6.5 CM have.  

Very cool gun, but you can literally move the barrel with your hands.  And yes, it's been back to the factory, they replaced the barrel and then said this is as good as it gets.  The issues are multiple. With gas system in the wrong spot, trunion too weak and anchored in a strange way, and barrel profile with thin section in an odd place.  

That said, I like 6.5. I like bullpups.  This is the only game in town.  What I really want is a light KelTek in 6.5 Grendel - thats not an option.  The MdrX is actually quite reliable, and just a cool fun-gun to shoot. And it looks cool, so there's that :)  It's a good gong-range gun, but not so great a bullseye gun. Good enough for hunting, but not hunting at the full range of the 6.5CM round likely.

It has the craziest load specific POI shifting as well (also horizontal).  Where you have to change zero by 6+ MOA depending on what ammo being shot.  But it does that repeatable at least.  It's spmething shootong one load at 200 yards, then switch ammo, look up the reference zero dope, click over a full 2 MIL per prior records (thats a lot), and first 200 shot be the same impact on target, with the different ammo.  That's the MDRx.  Recoil isn't bad, a good bit better than same weight ar10 6.5 cm, that part is good

So anyway, yes, my question was if I greatly decrease the strain on the system (gas, trunion, et al), will I decrease the extent of shot to shot variation, and decrease the incident rate of SUPRISE 4 MOA horizontal flyer?  (Heh and no, I'm not yanking the gun off by 4 MOA on the rest somehow).  The answer... maybe, but I didn't exactly throw a lot of 1 MOA tight groups to prove it. Will study the targets when get more time tonight.

OP:
Ok, If I understand your challenge:  that is same rifle/same barrel but when shooting different ammunition, you have a vastly different zero.  
Correct?  

Then FWIW I completed this test here:
https://www.ar15.com/forums/AR-15/200yd-Groups-Velocities-of-Winchester-62OT-mk318-64-GD-and-others-/16-744061/?page=1&anc=bottom#bottom

And I shot, in this case Win 62OTM and MK318 side by side.  
Same rifle same zero ............ and got a 5MOA total group with a 2MOA separation between the 2 loads.  
So your differences aren't that much different.

But if you have good zeros (and so can adjust between the different ammo) who cares?  

Unless you can make some adjustments to the rifle (ie ensure the barrel is fully free floated) and the Company has checked it, aren't you pretty much done?
I mean I would look for some mechanical issue and not mess with the ammo.  
Because if nothing else that ammo is married to that particular rifle.  
And if you are going to do that, why not buy/use/have one load for the rifle and be done?

He's changing calibers on a gas gun. There are just so many variables.

Ok, well if that is the case, then POI shifts are inevitable and solely dependent on the system of lock up.  
Changing of loads etc to get them closer is purely luck and down loading the ammo to hope to do that foolish as it degrades the effectiveness of the ammo.  

I had gotten too wordy - my bad.  Here is the root issue I an faced with.   This is prior data:

Here is a 20" AR10 group size performance and point of impact shift overlay graphic, for a spectrum of 6.5 CM ammo



Here is the same test done several months ago, with a 20" MDRx.  These tend to be "best 5 shots groups of the day" picked for the group size grapghic.



As you can see,a very wild change in POI based on ammo load.  Not shown well in this graphic, is the MDRx will also at times throw a wild wide horizontal shot, beyond what one might anticipate.  Another MDRx user with a 16" .308, inspired by this analysis, repeated and presented graphical his findings, which were also similar in behavior. But .223 MDRx users do not appear to have this complaint.  Same gun, just different bolt and BBL.  You can even swap back and forth if have both parts (I dont)

So, my hypothesis question: will a 6.5 CM MDRx will have better group size and fewer wild horizontal shot, if fired with weaker loads?  I thought it would.  Did it?  Eh, still analyzing, but I wasn't buying ice cream and all smiles on the drive home.  

Btw, for reference- here is what a factory RPR did

So why on Earth bother, and not just sell it?  Eh, I could - but I like 6.5mm, and I like bullpups.  So this is my only option.  And what an option it could be, if I can make it sing.  A 20" BBL 6.5 CM firing 40 gr of gunpowder per shot that's shorter than an M4, that you can CQB with, then drop down and firing that 20" BBL out to 1000+ yards with 6.5 CM.  That's something seriously profound.  And it is profound, but you really better know your gun an ammo, with this one.  And even with that, I've been able to ring a small silhouette gong 16 out of 20 shots at 500 yards, which isn't bad actually, but who knows where those other 4 shots went - the could have easily impacted as much as 2 foot or more away.  

But if I take it to a Bullseye Match, and try to shoot 500 yard high-power with it, this is what I tend to get:


Not too bad with the 85 gr Varminters; but notice two of those shots just went wild.  Horizontal left.  That's typical of the gun, and that wasn't wind.

Go heavier to 140 gr proper bullets for 500 yards, and I get this.  Very ugly:


So, can I downgrade to a mild 129 gr and sort of tighten it up and still get decent distance beyond 500 yard performance and terminal performance? (the point of this thread)   Analyzing still on that question.  Maybe, but only some.

OP:
Ok, great.............  

Just my 2%........  

In my limited experience and what I've seen:
Rifles that you can change the barrel (caliber) on, always result in the POI moving around.  
There is no way around that.  The question is how much.  


In my testing of alot of various ammo in the same rifle (ie the test I linked.... same 223 upper.....same zero, different ammo)....
You may or may not see an overlap.
(in my case and using theWIN 62 and mk318 as an example that was ok, one was for training and the other SD use, and good enough for my purposes).

In YOUR example (the second graphic) I'd use the ones that DO overlap....  
Because the issue, IMHO, is the rifle, not the ammo.......
and ignore the ones that don't.  

What I would NOT do is try to down load ammo to match another, you have enough different loads that do overlap to work.  
Especially the long-er range ammo, you want the velocity for a number of reasons.
Those reasons are more important than trying to match zeros.  

The issue of horizontal stringing is based in three things:  You, the rifle or your ammo.
You swear its not the first or third..........  
That leaves the rifle............
So don't "fix" the ammo to address a "rifle" problem.    



   

OP,

Have you reached out to ColdBoreMiracle on the hide? He does a lot of trouble shooting for DT and is well versed on the MDR series of rifles.

I’d be curious of his thoughts. With the stringing being directly horizontal, I’m curious if part of the bolt/locking mechanism is putting stress on a particular side of the barrel.

Yes, this right here...............
If the rifle is the issue...........fix the rifle!

The issue of horizontal stringing is based in three things:  You, the rifle or your ammo.
You swear its not the first or third..........  
That leaves the rifle............
So don't "fix" the ammo to address a "rifle" problem.