Help Choosing OCW (.308 Tikka CTR)

[Gtscotty]

I finally made it out to the range to try some new loads in my Tikka CTR today. To date I've been using 168gr Nosler HPBT's over IMR 4064, with good results, but decided it was time to branch out a bit. Today I shot Optimal Charge Weight workups for 178gr Amax over Varget, and my normal 168 Nosler HPBT, also over Varget.

For those of you who have some experience with OCW load workups, I'd appreciate your opinion on which charge weight you would choose to from my set of targets to pursue. I tried to mark a red dot where I figured the center of the group would be.

I'm leaning towards 44.3gr with the 178 Amax, and 45gr with the 168 HPBT, but I'd like to hear what others would do.

178 Amax, Varget


168 HPBT, Varget

 

[Gtscotty]

No opinions? Does anyone else use the OCW method?

 

[LNK]

No opinions? Does anyone else use the OCW method?

Kinda hard to beat that 43.7 gr with the 168's.

 

[LNK]

Kinda hard to beat that 43.7 gr with the 168's.

Sorry. 43 grain load.

 

[Blackrock]

Looks like anywhere from 43.3 to 44.3 is getting good results. Do you have a Chronograph or access to one?

 

[Ifishsum]

I agree with your own conclusions; both loads on either side of the ones you chose are also very good, which usually means you're in the center of an accuracy node. More tolerant of small variations in conditions and consistency.

 

[<*(((><]

Disclaimer: I have not yet utilized this method but have plans on using it soon on a .308. But have read and understand what the OCW is all about. Just wanted to be clear I don't have any first hand experience so take what I say for what you will. I'm assuming you are following Dan Newberry's method? I'm ignorant to if there is more than one method.

I would agree with your assessment, the loads on both sides of those are better than any of the others.

It looks to me like on the 178 AMAX over 44.3 of IMR 4064 your further tweaking should be up in powder charge towards 44.6 as both (44.3 and 44.6) are moving in the same direction POI. The 44.0 charge deviated from the other movements.

For the 168 HPBT over 45.0 of Varget it looks like similarly for further tweaking you should be moving towards your 45.5 powder charge.



Thank you for posting this as it is in my near future of performing the same test. Could you give some details on the rifle? I.E. Barrel length, twist.

 

[Gtscotty]

Looks like anywhere from 43.3 to 44.3 is getting good results. Do you have a Chronograph or access to one?

I agree that pretty much throughout that range, the center of each group doesn't move around too much, which is what you're looking for in OCW. That in and of itself made interpretation a little tougher though, as with other lighter rifles that didn't shoot quite as well, I've seen the center of the group move around more, so it was more obvious when three groups stayed in the same area... I do have a chronograph, but temps were around 20 degrees with 10 - 12 mph wind, so I was hold up in the shooting house. I'll probably chrono loads next time to see what I'm working with. For reference, I was getting right at 2600 fps with 43.5gr IMR4064 out of my 20 in barrel. I'd be happy with that or, preferably a little faster.

More tolerant of small variations in conditions and consistency.

That's what I'm going for, I've used other methods, but I like with the concept of OCW, and have had it work well before.

It looks to me like on the 178 AMAX over 44.3 of IMR 4064 your further tweaking should be up in powder charge towards 44.6 as both (44.3 and 44.6) are moving in the same direction POI. The 44.0 charge deviated from the other movements.

For the 168 HPBT over 45.0 of Varget it looks like similarly for further tweaking you should be moving towards your 45.5 powder charge.

That's an interesting idea, I might try edging my chosen loads up a bit to see if I can get them into the most variance tolerant space before I play with seating depth. I actually think I pulled the 44gr/Amax load down a bit, it might have lined up with it's neighbors better otherwise.

The rifle is a stock Tikka CTR with a 20 in, 11 twist barrel and SWFA SS 12x42 scope. The 168 HPBT's are actually Nosler blems from Shooters pro shop. I was surprised how well they did, I'm sure they are not all from the same production batch, and all I did was sort them by weight into .3 gr groups. I had burned through most of my original purchase of 400, so I went ahead and picked up 400 more last night. Thanks for the input everyone, it's good to get some confirmation of my interpretation of this workup.

 

[Gtscotty]

A strange thing happened tonight, I was looking over the CTR, and noticed the action screws were loose.... Like turning with my fingernail loose. The last time this rifle was handled was when I shot the OCW workup, so they must have been loose then as well. Tikka doesn't give action screw torque specs, so I just torqued it to 40 in-lbs. On one hand, I'm really surprised that the rifle shot as well as it did with loose action screws, on the other hand, I'm wondering if retorquing my action will throw off the results of my test?

 

[Bart B.]

I think both your group sizes and zeros will change by snugging up those stock screws.

Regarding the OCW method ......

If the one you're using is this one:

http://optimalchargeweight.embarqspace.com/

I think it's a figment of their imagination. Here's why.

First off, I think there's two shock waves; one starting at the front of the cartridge and the other at the back. He doesn't say where it starts from so there's no way to figure out when either one reaches the muzzle. If it starts in the middle of the cartridge where the powder is as I think, it goes both ways; forward and backwards from that point. And each one will make several round trips back and forth in the barrel before the bullet reaches the muzzle.

Second, speed of sound in steels has a range of several thousand fps. Most sites say around 16,000 fps in stainless steel. Not the 18,000 fps he states.

Third, the bore and groove diameters will change a few millionths of an inch depending on where one of those shock waves are. High power rifle competitors often clamp a band around the muzzle for front sights to be mounted on. That reduces the bore and groove diameters, too; moreso according one who air gauged barrels with and without that band on them. That barrel showed no change in accuracy with or without that band clamped tight on it.

Forth, if true, then barrels of different lengths would not shoot the same ammo equally accurate. But they often do.

Fifth, the most accurate match grade 7.62 NATO Garands would test good lots of commercial match ammo inside 4 to 5 inches at 600 yards. After 2000 rounds of barrel wear at the muzzle increasing bore and groove diameters from cleaning rod wear a thousandths or two, they still shot as well as bolt guns shooting that same ammo that much at 5 to 6 inches; both barrels had the same amount of throat erosion. But the bolt gun barrel had no wear at its muzzle; copper wash all the way to the crown. The Garand barrel had no copper wash the last half inch of the bore.

Nobody's proved OCW works with a timer to measure bullet exit time versus pressure from a strain gauge at the muzzle showing the time the barrel expands and contracts by that sound wave; if it's even measureable.

 

[Gtscotty]

Bart B,

I hope group sizes don't suffer from torquing the screws. Logically, I would expect the opposite, but the rifle shot quite well last weekend. I tried a few different torques in my Tikka Superlite and it showed a preference for ~50 in-lbs... it sure would be nice if Tikka would just publish a recommended torque. I should have checked the screws before hitting the range, but honestly, I always torque my action screws and I've never had this happen on a rifle before... I guess there's a first time for everything

What is your method for working up loads? Serious question, you obviously have a lot of match shooting experience. I've used OCW for a few other rifles and it served me well, but ballistics is not my field of engineering expertise, so I've never actually tried to verify the calculations myself. That said, I don't know where you are seeing 16k fps, a quick search of a couple NDT company websites, and engineering toolbox (which is usually good for a ballpark anyway) shows averages of 19k fps to 20k fps, with shock wave velocities in carbon steel being a bit faster than stainless. I don't have my MERM or Mark's handbook on me, but I can see if they have anything more definitive tomorrow.

I've looked at the ladder test, but the idea of having a sample size of one at each charge weight seems a bit suspect to me... I'm not sure I'm good enough for that.

I just finished loading up different OAL's at 45gr for the 168's and 44.3gr for 178's, hopefully I'll get a chance to try them Sunday or Monday.

 

[Gtscotty]

As an aside, here is an interesting FEA analysis of a rifle during firing. Their conclusions that the recoil, and subsequent torque induced deformations have much more of an impact on muzzle direction at the time of bullet exit than shock waves and natural frequency vibrations make a lot of sense.

http://www.varmintal.com/apres.htm

 

[Bart B.]

GTScotty,

One site on speed in steel:

https://books.google.com/books?id=K...page&q=speed of sound in steel 16,000&f=false

Geoffrey Kolbe (Border Barrels) has a web site that uses FEA software to predict where the barrel points when bullets exit. Put in your barrel dimensions and how far its muzzle axis is above the center of mass holding it and the results show the arc the bore axis at the muzzle points while the bullet goes through it. Pick the barrel time point your bullet exits and that's the angle it leaves at compared to where the axis was before the round fired:

http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/barrel_vibrations.htm

Depending on the site seen, there's a 3000 fps spread in speeds. That's my main point.

I've never worked up any load except for one new Sierra match bullet. Along with a few others, we settled on 45.3 grains of IMR4895 under Sierra's new 30 caliber 155-gr. match bullet in new unprepped cases save uniforming their inside neck diameters. It tested 20 shots inside 3 inches at 600 yards and shot 3 to 4 inches at 600 in a couple dozen or more rifles in the first match it was used in according to the people who used it. All sorts of bore, groove and chamber dimensions in match grade barrels from around the world.

I use the same loads as those shooting the best scores. Tests with them show they're as accurate as anyones. They convinced me not to wear out a barrel developing a load. Get a good barrel installed by someone who's good then shoot the darned thing; no break in needed. Stay far, far away from ball powders if accuracy is your first objective.

Yes, a sample size of one for each charge weight is not a statistically significant idea. But it's great for emotions when one thinks the first shot of any group will always be in the middle of it.

If you choose to work up a load testing several few-shot groups each with different powder charge weights, pick the load whose largest group is the smallest. That shows what happens when all the variables tend to add up in different directions. The smallest groups for any load happen the vast majority of time when all those variables pretty much cancel each other out. Rarely, if ever, do they happen when all the variables are minimum. Best accuracy happens when all the variables have the smallest range. Us humans usually have the biggest variable as we hold onto the rifle pulling it back into our shoulder; don't forget that.

 

[Gtscotty]

FYI, I checked today, and the MERM cites the speed of sound in steel as 16,900 fps, whereas Marks handbook specifies 20,013 fps. Neither of the books go to the detail of speed in different types of steels, but, at any rate, 18,000 fps is a reasonable ballpark figure to throw around. All of that said, I agree that the deformation of the barrel due to recoil forces, and the associate torque will very likely have much more of an impact on muzzle direction at bullet exit than small amplitude oscillating shock waves and vibrations, so it's kind of a moot point.

Do you have any universally successful loads for 168gr and 175gr similar to the 155gr load that you posted.

 

[Andrew Leigh]

@GTscotty.

The one criteria in OCW analysis is that the three shot represent an isoceles triangle. Vertical and horizontal stringing are classic signs of not being on a node.

Another good sign of being on the node is that when bisecting all sides of the triangle (clover leaf) that the intersection point of all is within the bounderies of the clover leaf.

Then the intersection point should be the one closest to the point of aim. So if you have two groups at near equal size and shape take the one closest to your aiming point.

Finally, try set you scope so that you are an inch or two high or low. The avoids shooting away the aiming point which may lead to poorer shots.

Nice shooting.

 

[Gtscotty]

Ok, here is the follow-up using my OCW-derived charge weights and varying OAL's. Today wasn't a great day to shoot (15 -20 mph winds from 10 oclock) but it was the only day I had.

With the 168's, my measured OAL to the lands was 2.88", so I started at 2.85" and worked up. 2.85" isn't shown because they didn't land on the target... I shot 5 rounds at the 400 yd steel plates to foul the bore and warm up, but then forgot to dial my elevation and windage back... rookie mistake. I'll be sticking with 2.86" on the 168 HPBT, which is coincidentally the OAL that worked best with IMR 4064 as well.



With the 178's, my measured OAL to the lands is right at 2.9". I'm definitely going with 2.885", which is probably one of the best 5 shot groups I've ever shot. Overall the Amax's shot better, but that might be because they were production grade, and the 168's were blems. Also the last target is totally superfluous, by the time I shot it, the wind had ripped the staples out of the top left corner and folded the paper over on itself. The only aiming point I had was the bottom center of the grey square (instead of the middle) and I think that threw me off elevation-wise on a few shots.

 

[snuffy]

Best accuracy happens when all the variables have the smallest range. Us humans usually have the biggest variable as we hold onto the rifle pulling it back into our shoulder; don't forget that.

Most of what Bart says is spot on. But that last statement should be the most important part. I will add that the rest you shoot off of should also be part of that equation, or at least be highlighted in the discussion.

How many people say they got this or that group when shooting their rifle, then when asked what rest did you use, they say my jacket rolled up on the hood of my car!

OCW or for that matter the "ladder" test is mostly smoke and mirrors. Unless the rifle is fired from a machine rest, with no human contact, you won't get the true picture of what the load is really doing. Some shooters are real good at shooting from a real good rest because they know to do it with consistency each and every shot.

Another variable is the weather. Perfect days do not exist outdoors. There's always something, wind, variable sunlight, temperature, and humidity. Impossible to duplicate conditions.

The aiming point is also of great importance. It should be clean and big enough to be easily seen through the scope. If the scope DID NOT cost nearly as much as the rifle, you will have scope induced inaccuracy. Get better glass!

I don't have a target rifle, at least not any more. I shoot factory rifles, try to get the best performance out of them. My method is as old as the hills, load a series of loads and even powder increments from the recommended starting loads, up to max, usually 5 rounds each. Those are fired from a tripod rest with a rear rest @ 100 yds. I do not test on windy days. Yeah I know waiting for a better day is hard, especially if you're eager to see what that rifle will do.

 

[Gtscotty]

How many people say they got this or that group when shooting their rifle, then when asked what rest did you use, they say my jacket rolled up on the hood of my car!

Anyone who can shoot similar groups off the hood of their car is way better than I am. As I stated in an earlier post I was shooting out of a shooting house so as to be protected from the wind. Specifically, I was shooting off of bags front and back on a solid bench.

OCW or for that matter the "ladder" test is mostly smoke and mirrors. Unless the rifle is fired from a machine rest, with no human contact, you won't get the true picture of what the load is really doing. Some shooters are real good at shooting from a real good rest because they know to do it with consistency each and every shot.

I guess my only question is how do you know you can't get a reasonably accurate picture of what the rifle/load is doing with a fairly consistent shooter? Isn't that the goal of any load workup? I'm aware of the variables introduced by the shooter, I'm just not sure that with a reasonably good shooter, their presence totally negates the efficacy of something like the OCW method. I see some merit in the OCW method, possibly not for it's stated reasons, but that I think there may be something to trying to time the bullet's exit from the barrel. Given that the barrel flexes and curves a small amount under recoil before the bullet exits, as shown in the FEA analyses above, the muzzle of the barrel will trace an arc. If you could time your bullet exit to coincide with the inflection point at the top of the arc, you would have a significant length of time (relative to the time scale we are talking about) where small changes in the bullet's exit time would correlate to very little difference in muzzle direction. It would be interesting to run an FEA analysis on my barrel (or as close of an approximation as you could get), and then try to figure out the timing on when my bullets are leaving my barrel. At that point it might be possibly to try and figure out if the OCW loads leave the barrel approximately at it's inflection point.

The aiming point is also of great importance. It should be clean and big enough to be easily seen through the scope. If the scope DID NOT cost nearly as much as the rifle, you will have scope induced inaccuracy. Get better glass!

I like these targets because they are free, and allow for a very precise aiming point. I'm using an SWFA SS 12x42. Not an expensive scope, but very repeatable, and the glass is definitely good enough to see the detail on my targets at 100 yds. If I were going primarily for benchrest groups I would get something higher power and fancier, but my main goal is to work up a reliable load to use at the monthly precision rifles shoots at our range's 700 yd bay.

As for the wind, it was a bit of a hindrance, but honestly, significant wind will be a constant here until about June, and I don't care to wait that long. You've got to do what you can with what you've got. At least it wasn't 50 - 60 mph like it is some days!

 

[Nature Boy]

I was going to try the OCW method this weekend but you guys have me rethinking it. I have some very accurate loads so far but I've more or less stumbled onto them

 

[Gtscotty]

Might as well give it a go and see if it works for you. Whatever the science behind it, i think i wound up with good loads for my rifle. If you try it, we can add another data point to the discussion.

 

[Nature Boy]

I'll do it scotty. It will be interesting to compare. I have enough 178 AMAXs and Varget to do it.

I'll load them up tonight

 

[Dave P]

I liked the original 168 and 43 varget.

If I was in your shoes, I would shoot 10 round ladder of that load: like from 42.5 gr to 43.5 grain. All aimed at the same point. 200 yards.

Then look for a group of maybe 3 shots that clump together. That is your OCW, that allows for variations in velocity and charge weight, while still hitting the X.

I swear by this method for an accurate, scoped bolt gun. Also worked on my match AR as well. Never had any success with M1 or M14 - too much stuff hanging off the barrels.

Yes, you have to be an excellent bench shooter to make this work. I believe this is the Creighton Audette method (may be a variation). No smoke and mirrors in my tool box.

And once you fire those tens rounds and hopefully get that tight clump - you are done! Leave the OAL alone.

 

[Andrew Leigh]

Might as well give it a go and see if it works for you. Whatever the science behind it, i think i wound up with good loads for my rifle. If you try it, we can add another data point to the discussion.

It is important to understand the evolution.

Dan Newberry created the OCW theory which is a very efficient means of establishing loads. He and certain Chris Long are friends and they were discussing why the method was effective. Chris having an acoustic engineering background I think it was, then set about trying to get the theory to line up with the practice so that one could become more predictable. So the fitted a rifle with sensors and proceeded to throws much lead down the range and they gained data. Chris then established formulae that would express the OCW relationship and allow one to predict accuracy nodes before shooting away too many rounds.

The gave rise to the OBT (Optimal Barrel Time) and the concept of accuracy nodes. The theory is laid out in Chris Longs website and make for excellent reading.

The OBT theory is well known to QuickLoad users as it is the key target that one aim for when developing a load. So the OCW method you subscribe to is sound and very effective. The next step if you have the money would be to buy QuickLoad but you would also need a chrony. One can develop accurate loads in as few as 8 rounds.

If you continue with the OCW then PM me an e-mail address. I developed an Excel Spreadsheet based on Dan's site which make OCW even easier.

 

[Bart B.]

If the OCW reasoning is valid, then will the same charge weight for different lots of the same make/type of powder be best for accuracy for all other components of the load not being from the same lot?

 

[Nature Boy]

I'd love to have that spreadsheet Andrew. PM sent