Understanding "Nodes"

I've been shooting a 6mm CM with a new barrel. It was producing 40-50mm 100M groups with a stock standard load (41.5 AR2209 behind Sierra 110's - and various similar quantities of AR2209). Gunsmith is standing right behind his work and has checked it, re-crowned and supplied ammo to test, top service.

Last night I shot it with a very small sample of 110's the GS supplied with 39.5, 40.0 and 40.5 of H100V. Bingo - groups of 11, 9 and 18mm at 100M. I didn't crony these loads but the GRT tells me the velocity would be about 2870 fps (ish) from my 24" barrel so not a million miles away from some of the AR2209 loads I've tried.

I'm struggling a bit to understand this, I've not previously encountered a rifle so "sensitive" to a load. I found this old thread ( https://www.nzhuntingandshooting.co....de-yours-6525/ ) which was useful but I'd like to get a bit better "frame of reference".

How common is it for a rifle to go from 50mm groups to sub-moa with a simple change of powder?? Is this an example of "finding a node"??

Sounds like the only variable is the velocity, but whatever the reason, you've stumbled on a great recipe.
The chrony will confirm if the speeds are close, which I suspect at 40gr they are

It is all to do with barrel harmonics, different powder bullet combinations create different vibrations you have to find the combination that suits your barrel
changing bullets and primers can alter the harmonics as well, A friend of mine is very anal about this when he gets a new barrel he buys enough components
to last the expected life of the barrel so there is no batch to batch veriation,
If you really get into the Node hunting game it will drive you crazy and broke, Google barrel tuners the Benchrest crowd use them mainly on .22 Rimfire where you can't
tune the load they tune the barrel vibrations, some do use them on centre fire try yours with a suppressor that will damp down the vibrations it may or may not help.

In my 243 I was having trouble getting it to shoot well. I put pillars in and free floated the barrel and it still sprayed the bullet I was using at the time, finally I tried 95 gr SST and BTips and got it shooting around 14 to 19 mm groups this was with AR2209. I tried changing the powder to Superformance and the groups opened up somewhat.
It's been a fussy rifle but when you find what it likes it a good shooter.

@Tentman, couple of questions

So all 3 the loads (39.5, 40 and 40.5) produce significantly better groups with H100V than 2209 ?

What speed does 41.5 of 2209 give you ? Compared to the estimated 2870 with H100V ?

Was seating depth for the H100V rounds the same as for the 2209 loads ?

Some general comments from my side:

- I look for speed consistency first, using a good (non optical) chrony, and ignore group size. Once I find a good SD value for 5-shot strings over the chrony, I play with seating depth to shrink group size.
- It would be strange to find a "node" that is as wide as a full grain for a medium sized case like the 6CM. When looking for nodes I tend to use increments of 0.3 or 0.4 (around 1% of volume).

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Originally Posted by ebf

@Tentman, couple of questions

So all 3 the loads (39.5, 40 and 40.5) produce significantly better groups with H100V than 2209 ?

Yup - hugely better, about the best it'd shot with 2209 was 40mm, with H100V the first group was 11mm

What speed does 41.5 of 2209 give you ? Compared to the estimated 2870 with H100V ?

2940's

Was seating depth for the H100V rounds the same as for the 2209 loads ?

Yes

Some general comments from my side:

- I look for speed consistency first, using a good (non optical) chrony, and ignore group size. Once I find a good SD value for 5-shot strings over the chrony, I play with seating depth to shrink group size.
- It would be strange to find a "node" that is as wide as a full grain for a medium sized case like the 6CM. When looking for nodes I tend to use increments of 0.3 or 0.4 (around 1% of volume).

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Yeah I usually do too, but when its a "precision rifle" with a high quality barrel done by a 'Smith who has already built 9 rifles on the reamer without an issue, the wheels fall off a bit. I run a Magnetospeed but stopped hanging it off the barrel when it wouldn't griup - no good giving it "excuses"

I understood all of these questions a lot better after watching some links to Eric Cortina posted on the forum, very helpful

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Originally Posted by Mooseman

In my 243 I was having trouble getting it to shoot well. I put pillars in and free floated the barrel and it still sprayed the bullet I was using at the time, finally I tried 95 gr SST and BTips and got it shooting around 14 to 19 mm groups this was with AR2209. I tried changing the powder to Superformance and the groups opened up somewhat.
It's been a fussy rifle but when you find what it likes it a good shooter.

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The 243s iv had were a bit fussy ive found they like the lighter bullets i think my current one likes the 85gr sierras thumnail sized groups are the norm for that one

Quote:

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Originally Posted by Moa Hunter

I understood all of these questions a lot better after watching some links to Eric Cortina posted on the forum, very helpful

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Haha - I've watched a lot of Eric's vids and was going to follow his methodology exactly for this rifle, however like a lot of internet stuff, the vids only cover "how to do it" when everything is sweet, once there is a problem, the answers are not covered. This isn't a criticism of Eric's stuff, there are a lot of variables so I wouldn't expect him to cover off every eventuality . . . .

Quote:

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Originally Posted by Mauser308

From experience of things that go bang in mining blasting, different rates of combustion produce different peak pressures but overall gas production as a factor of time remains consistent (relative to cracking rock, heave or rock throw remains similiar but the difference is in the gauge or size of the shattered rock after the shot is fired - faster peak pressure equals more shattering effect everything else being equal). Possibly, just possibly there is a varience of where the powder burn is producing the peak pressure in the barrel which suits the bedding and bolt setup (lock time etc). Unless you must absolutely use ADI powder, you've found the answer. If you must use ADI powder, shrug, can't help...

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Ladder test velocity shows charge weights with more consistent velocity spread and conversely a charge weight slightly higher or lower with more velocity spread. My 'New Messiah' Eric tells us that this is all down to combustion factors

If you run a saw blade between your fingers, or some other wippy thing, you will feel the effect the projectile has in the barrel as it travels through. The end of the barrel is wipping about. You want the projectile to always meet the muzzle at the exact same point in the barrels movement and you want the barrel to move the same way every time. Lots of ways to achieve that and also lots of things that influence that motion.

I became obsessed with nodes. All my reloading is about nodes. I shoot my load development off a field rest because of harmonics. Drives people mad.

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Eliminating variables . . . bloody hard with rifles. It is shooting a lot better with AR-2209. I used GRT to see what the predicted velocity of the H100V loads were, then "matched" that in 2209. Worked pretty good and its shooting about 0.5 MOA (with the Sierra 110's) but there are occasional "loose" shot - which there didn't seem to be with the H100V. I've got a litle bit of H100V to try some more groups with to see if they also exhibit the "loose" shots syndrome. I have the feeling from the 9 rounds of it I fired (always dangerous territory, just about as bad as people who say "I believe") that the H100V had a wider more generous node.