Cold Bore Shots, POI Shifts, and Canted Scopes

[AM_Hunter]

Hi Everyone,

Im trying to better educate myself on long range shooting, not necessarily to take game at further distances but to just become a better overall shooter. I read through most of the recent threads on cold bore shots and based on what ive read it seems the consensus is that cold bore shots dont significantly shift your point of impact.

Last weekend I did a little long range shooting with my 300 PRC, shooting Hornady 225gr Match factory ammo out of a 26" carbon fiber barrel. It had been a little bit since I last shot the rifle but the barrel was not cleaned since the last time I took it out. The box specs put the fps of the round at 2810fps, my first shot was 2 MOA high with a chrono read of 2796 fps, no adjustments made to the dial. My second shot was about 1 MOA high and a chrono read of 2749fps. My 3rd-5th shots were right at zero with fps at 2688, 2675, an 2655fps. The change in fps correlates to the change in POI, and from what Ive read the velocity tends to decrease as the barrel gets hotter due to gas escaping, but it still seems like a significantly large fps change, even for factory ammo. Additionally, when I threw an average velocity of 2672fps into my ballistic calculator it was giving me MOA adjustments that were a lot higher than reality. For example at 800 yards I was adjusting about 16.5MOA to hit the target but my calculator was saying over 19 MOA at that lower fps.

Any thoughts on why the fps readings were so low and if the hotter first round was due to it being a cold bore shot?

Could the chrono be misreading the shots? My buddies fps numbers were coming in lower than he expected too.

When I adjusted the fps in my calculator back to 2810fps the moa readings I was getting were still about .5MOA - 1MOA higher than what was needed, which makes me believe the round was traveling faster than 2810 fps, unless it was due to something else? I tested this with two different calculators and they both had similar MOA adjustments, but were higher than reality.

Like I said I am just trying to get a better understanding of everything and figure out if what I am seeing is normal or if theres an issue causing it that I could potentially remedy.

On a side note, how significant is the effect of a not level scope? Part of me is thinking my scope is not perfectly level. Or my scope bubble is off. My wind adjustments were anywhere from 1.5-2 MOA left with almost no wind at 600-1000 yards but my calculator showed it shouldnt be more than .6MOA at 1000, which makes me think my scope is potentially not level...

Appreciate any input!

 

[Harvey_NW]

300 PRC is a lot of recoil to deal with, even with a brake. I wouldn't put a lot of merit into those results from only a couple shots, you might be perceiving it that way but a lot of times velocity variation doesn't translate into POI shift exactly the way the numbers line up. This is why chronographs are a decent reference tool, but you need to true your calculator. The way I understand it, closest to transonic is best. Make sure all your other inputs are correct (scope height, box BC, etc.) then true it up at 800 yards at a waterline, the velocity is what it is once your vertical is centered up. It is possible the chrono was misreading, what's important is what happens on target. Factory ammo, round count, and other variables might be contributing to the ES if those are the correct readings, your barrel just might not like that powder.

Canted scope would cause some left/right deviation, easy to check with a plumb line. I use paracord and tie something with a little weight on the end. It's a little more bold.

 

[Scottf270]

I'm not up on the newer chronos. I have a 25 year old Chrony model with the sticks and light diffusers. It unfolds and mounts on a tripod. I noticed if it wasn't level I would get different readings. Yous may be different.

 

[eric1115]

Did you confirm 100 yard zero?

 

[AM_Hunter]

300 PRC is a lot of recoil to deal with, even with a brake. I wouldn't put a lot of merit into those results from only a couple shots, you might be perceiving it that way but a lot of times velocity variation doesn't translate into POI shift exactly the way the numbers line up. This is why chronographs are a decent reference tool, but you need to true your calculator. The way I understand it, closest to transonic is best. Make sure all your other inputs are correct (scope height, box BC, etc.) then true it up at 800 yards at a waterline, the velocity is what it is once your vertical is centered up. It is possible the chrono was misreading, what's important is what happens on target. Factory ammo, round count, and other variables might be contributing to the ES if those are the correct readings, your barrel just might not like that powder.

Canted scope would cause some left/right deviation, easy to check with a plumb line. I use paracord and tie something with a little weight on the end. It's a little more bold.

My first thought was that the POI shift was due to pulling but the way it lined up with the FPS made me think otherwise. I was trying to true the calculator based on my results that day but haven't been able to, I have triple checked my settings in my calculator and they are all correct as far as I know. The only thing that got me close to the results I was getting out there was setting the fps of the round to 2850fps, which made it more accurate at the lower distances but then under at the further distances. I think the rifle likes the factory ammo im using based on the group I got at 100 on rounds 3-5 and the fact I was able to hit 1000 4/5 shots. But clearly I need to do more testing at 100 and more shots with a chrono to get more data.

Can you explain a few of the things you mentioned? Like:

"True it up at 800 yards at a waterline"

"Closest to transonic is best"

Thank you!

 

[AM_Hunter]

I'm not up on the newer chronos. I have a 25 year old Chrony model with the sticks and light diffusers. It unfolds and mounts on a tripod. I noticed if it wasn't level I would get different readings. Yous may be different.

That could be possible, will have to do more testing with the chrono

 

[AM_Hunter]

Did you confirm 100 yard zero?

Yes, my first shot through the rifle, cold bore and all, no cleaning, was 2 MOA high. Made no adjustments, next shot was about 1 MOA high. Again made no adjustments, next 3 were all in the center under 1 MOA group size.

 

[willfrye027]

Yes, my first shot through the rifle, cold bore and all, no cleaning, was 2 MOA high. Made no adjustments, next shot was about 1 MOA high. Again made no adjustments, next 3 were all in the center under 1 MOA group size.

what does your 100 yard zero target look like?

You really need a decent sized group at 100 yards to confirm your zero. Ideally the bullseye is obliterated. That is probably the most important step in the whole process otherwise it’s impossible to know where the error is coming from.

 

[AM_Hunter]

what does your 100 yard zero target look like?

You really need a decent sized group at 100 yards to confirm your zero. Ideally the bullseye is obliterated. That is probably the most important step in the whole process otherwise it’s impossible to know where the error is coming from.

In the past its been all good, most shots in the bullseye, any misses were due to shooter error, flinching, and maybe barrel heat up. I only had 35 rounds to shoot that day and needed to shoot out to 1000 to confirm dope because we have a little long range shooting competition between some buddies and other guys this weekend but ill be sure to shoot more groups at 100 and see what happens. Im curios to see if I shoot 100 cold bore again if id have a 2 MOA shift again or if that was just me flinching.

 

[Harvey_NW]

My first thought was that the POI shift was due to pulling but the way it lined up with the FPS made me think otherwise. I was trying to true the calculator based on my results that day but haven't been able to, I have triple checked my settings in my calculator and they are all correct as far as I know. The only thing that got me close to the results I was getting out there was setting the fps of the round to 2850fps, which made it more accurate at the lower distances but then under at the further distances. I think the rifle likes the factory ammo im using based on the group I got at 100 on rounds 3-5 and the fact I was able to hit 1000 4/5 shots. But clearly I need to do more testing at 100 and more shots with a chrono to get more data.

You need to verify your other inputs are correct, most people jump to adjusting BC and something like scope height isn't correct. Use the advertised BC from the manufacturer and whatever velocity got your vertical centered up at 800 and start adjusting other inputs until things line up. And as mentioned, you need to verify zero with a large sample size.

Can you explain a few of the things you mentioned? Like:

"True it up at 800 yards at a waterline"

"Closest to transonic is best"

Thank you!

A waterline is a horizontal line across a target at the truing range, you're shooting to center up vertical so it's an easy way to line the reticle up consistently, and see impacts above and below. Disregard windage adjustments when shooting at a waterline. Transonic is where the bullet reaches 1,340fps and best for data points, but sometimes that's a bit farther than 800 and takes a pretty competent shooter to shoot consistent enough to get good info from.

 

[eric1115]

Yes, my first shot through the rifle, cold bore and all, no cleaning, was 2 MOA high. Made no adjustments, next shot was about 1 MOA high. Again made no adjustments, next 3 were all in the center under 1 MOA group size.

Gotcha, I read your post as having gone straight to distance.

If I were in your shoes, I'd go back to the beginning to confirm foundational stuff before trying to straighten out the stuff that's built on it.

1) What scope are you using? 2MOA shift that settles back to old zero after a couple shots is not unheard of.

2) action screws, base/ring screws torqued correctly and loctited?

3) 10 round zero group

4) 5-10 round group at a known distance (very important to get an accurate range measurement) far enough that velocity matters. That's pretty far with a bullet as slippery as yours. 800+ probably. Needs a real aiming point, and needs to be paper/cardboard or a big piece of freshly painted steel so group size and offset can be measured precisely. 20 rounds spent this way will tell you far more than 4-5 round zero check then 15 shots making adjustments to get 6-8 hits at 5 different distances.

 

[willfrye027]

In the past its been all good, most shots in the bullseye, any misses were due to shooter error, flinching, and maybe barrel heat up. I only had 35 rounds to shoot that day and needed to shoot out to 1000 to confirm dope because we have a little long range shooting competition between some buddies and other guys this weekend but ill be sure to shoot more groups at 100 and see what happens. Im curios to see if I shoot 100 cold bore again if id have a 2 MOA shift again or if that was just me flinching.

Gotcha. Personally I like to get 20 shots on the 100 yard target and a perfect zero. That group may be larger than you think it should..1.5-2 moa is not unheard of for a magnum. Either way it is what it is…from there you will KNOW if a shot is falling out of your expected cone of fire…hard to true at 1000 if you’re not sure it’s zeroed perfectly at 100 and if you don’t know what the true cone of that gun is.

 

[AM_Hunter]

You need to verify your other inputs are correct, most people jump to adjusting BC and something like scope height isn't correct. Use the advertised BC from the manufacturer and whatever velocity got your vertical centered up at 800 and start adjusting other inputs until things line up. And as mentioned, you need to verify zero with a large sample size.


A waterline is a horizontal line across a target at the truing range, you're shooting to center up vertical so it's an easy way to line the reticle up consistently, and see impacts above and below. Disregard windage adjustments when shooting at a waterline. Transonic is where the bullet reaches 1,340fps and best for data points, but sometimes that's a bit farther than 800 and takes a pretty competent shooter to shoot consistent enough to get good info from.

For the inputs on the BC ive tripled check them. What are the major contributing factors that could cause that?

BC is correct
Scope height is correct
Bullet diameter and length
Using Density Altitude, at 3400' elevation and 60F I get about 4000' DA
Barrel length and twist set properly
The only real unknown atm is the velocity but based on results it should be shooter faster than the box implies not 150fps slower based on the chrono ...

 

[AM_Hunter]

Gotcha. Personally I like to get 20 shots on the 100 yard target and a perfect zero. That group may be larger than you think it should..1.5-2 moa is not unheard of for a magnum. Either way it is what it is…from there you will KNOW if a shot is falling out of your expected cone of fire…hard to true at 1000 if you’re not sure it’s zeroed perfectly at 100 and if you don’t know what the true cone of that gun is.

Yea ill have to just put more rounds at 100 to be sure. Im fairly confident it is zeroed at 100 based on past shooting sessions and that last three shot group, its just those two initial rounds that threw me off, especially because I did not adjust anything after the shots and then it ended up on zero.

We'll see how I do this saturday at our competition given the dope I got from my last session and see if I can repeat those hits. We are shooting at cardboard with paper targets so ill also be able to get an idea of group size.

 

[willfrye027]

Yea ill have to just put more rounds at 100 to be sure. Im fairly confident it is zeroed at 100 based on past shooting sessions and that last three shot group, its just those two initial rounds that threw me off, especially because I did not adjust anything after the shots and then it ended up on zero.

We'll see how I do this saturday at our competition given the dope I got from my last session and see if I can repeat those hits. We are shooting at cardboard with paper targets so ill also be able to get an idea of group size.

Sounds like the two unknowns are perfect zero and velocity

 

[AM_Hunter]

Gotcha, I read your post as having gone straight to distance.

If I were in your shoes, I'd go back to the beginning to confirm foundational stuff before trying to straighten out the stuff that's built on it.

1) What scope are you using? 2MOA shift that settles back to old zero after a couple shots is not unheard of.

2) action screws, base/ring screws torqued correctly and loctited?

3) 10 round zero group

4) 5-10 round group at a known distance (very important to get an accurate range measurement) far enough that velocity matters. That's pretty far with a bullet as slippery as yours. 800+ probably. Needs a real aiming point, and needs to be paper/cardboard or a big piece of freshly painted steel so group size and offset can be measured precisely. 20 rounds spent this way will tell you far more than 4-5 round zero check then 15 shots making adjustments to get 6-8 hits at 5 different distances.

Ill have to double check scope ring torque specs. Action and base were mounted at factory and I havent touched them. Scope is a Vortex Strike Eagle 5-25x50 FFP

All my shots were on freshly painted steel at known ranges, 200, 300, 409, 570, 796, 999. Was able to get repeated hits on them.

Ill be shooting 5 round groups at cardboard this weekend from 200-1000 so Ill get a little better idea of all that as well.

I just want to gather as much information and knowledge as I can so I can get the most out of my shooting sessions.

 

[Harvey_NW]

Ill have to double check scope ring torque specs. Action and base were mounted at factory and I havent touched them. Scope is a Vortex Strike Eagle 5-25x50 FFP

Not trying to dog on your equipment but Vortex is known for having wandering zero and erector/tracking issues when dialing. Those issues get amplified at longer distance, and it wouldn't surprise me if that was contributing.

 

[AM_Hunter]

Not trying to dog on your equipment but Vortex is known for having wandering zero and erector/tracking issues when dialing. Those issues get amplified at longer distance, and it wouldn't surprise me if that was contributing.

Yea ive heard of some of the issues vortex's have had, ive considered it a possibility but only way to confirm is to dial out to the previous dopes I got at 400-1000 and see if its its still hitting, that as well as shooting it at 100 again to see if the zero has changed at all

 

[huntnful]

Those FPS differences absolutely would not account for a 2 MOA or even 1 MOA shift at 100 yards. So I wouldn't correlate them to the velocities gathered at all. So you can eliminate that for sure.

Next would be a legit huge cold bore shift. Possibly the case. Maybe factory rifle with poor, or no bedding, combined with the Carbon Barrel. Worth a double check, but still out of the ordinary.

Next would be the rifle just actually shoots larger groups than you think, and you haven't tested it extensively enough yet. Still a big maybe.

Next would the optic system. This is a highly probable combination to cause your shifts. Check the torque on your action screws first. Then the rail to the action. Then the rings to the rail. Then get a new scope that doesn't shift zero from driving down the road.

As far as your impacts not lining up at 800 yards. You need a LEGIT 100 yard zero, before you start inputting corrections into the calculator. Once the zero of at least 10 shots is established. Then shoot 800 yards and adjust velocity to match impacts.

 

[AM_Hunter]

Those FPS differences absolutely would not account for a 2 MOA or even 1 MOA shift at 100 yards. So I wouldn't correlate them to the velocities gathered at all. So you can eliminate that for sure.

Next would be a legit huge cold bore shift. Possibly the case. Maybe factory rifle with poor, or no bedding, combined with the Carbon Barrel. Worth a double check, but still out of the ordinary.

Next would be the rifle just actually shoots larger groups than you think, and you haven't tested it extensively enough yet. Still a big maybe.

Next would the optic system. This is a highly probable combination to cause your shifts. Check the torque on your action screws first. Then the rail to the action. Then the rings to the rail. Then get a new scope that doesn't shift zero from driving down the road.

As far as your impacts not lining up at 800 yards. You need a LEGIT 100 yard zero, before you start inputting corrections into the calculator. Once the zero of at least 10 shots is established. Then shoot 800 yards and adjust velocity to match impacts.

Seems to all come back to confirming my zero. Ill see if I can pickup an extra box of ammo and do a 10 round group at 100 friday night and be able to check to see if there is a cold bore shift or not.

Rifle is a Christensen MPR. I know theyve had QC issues but my rifle has for the most part shot 1moa groups at 100 in the past and shoots better than I probably can so I dont think its a bedding issue or action or rail issue but I wont rule them out. Could very well be a scope issue, I bought the scope back in 2021 before I really paid attention to tracking and drop testing. Itll require some more testing to be sure.

Appreciate the feedback!