Accuracy using MagnetoSpeed Sporter Chronograph

[macandrac]

I purchased a MagnetoSpeed Sporter Chronograph and have used it on two separate Range outings so far. The Chronograph was easy to attach to the end of my barrel (6.5 Creedmoor) and appeared to be working fine (although it never gets the 1st shot accuracy correct - think it uses this for calibration or something. I discovered a couple things that I did not realize initially; 1) having the Chronograph hung over the end of the barrel somehow affects the accuracy of my shots - perhaps this is well known but something I wasn't aware of and 2) when I shoot 3 sets of 20 rounds in succession (with breaks in-between), I notice the bullets velocity increases quite a bit from the 1st set to the 3rd set. I'm assuming the accuracy issue is because you are clamping this thing to the barrel and affect the barrel harmonics and/or the bullet's path over the 10" of Chronograph somehow affects its flight slightly. Regarding the increase in bullet velocity, I'm assuming this is a result of some light fouling that increases the pressure inside the barrel but not sure on this. I'd like to hear back from the experts out there on their take of what is going on.

Gun: Palmetto State Armory PA-65 Gen 3; 6.5 Creedmoor; 20" Barrel
Ammo: S&B 140 Gr FMJ
Range: 200 Yard Benchrest (shot both 100 Yard & 200 Yard Targets)
Location: Malabar, FL (PMRPC)

 

[Scott_at_Vortex]

IMO humble opinion and experience your "accuracy issue" is likely due to adding weight to the end of the barrel thus changing harmonics. Your velocity increase could be from a new barrel speeding up(if its new) and/or the gun getter hotter affecting powder stability thus increasing pressure/velocity.

Also, in my experience the magnetospeed has not affected accuracy when running a silencer and/or heavy barrels. Ive shot many PRS stages over the years with a magneto attached. You know, because primetime to confirm your MV is during the match.

 

[Sig Marine]

I would suggest looking into the Wiser mount. It attaches to your chassis and has a rod extending parallel to the barrel where you attach your bayonet thus taking it off the barrel and eliminating the POI shift

 

[R_A_W]

1) really isn't surprising at all, as @Scott_at_Vortex mentioned, you've modified the barrel's harmonics.

2) How much fps increase are you talking about? What is the round count on the barrel? How warm is the barrel for each of the 20-shot strings? It's August and you're in Florida... is your ammo sitting in the sun while you're shooting the strings?

You know, because primetime to confirm your MV is during the match.

Real pros challenge themselves maneuvering inside a bus with a 42" barrel with a suppressor & a MagnetoSpeed attached

 

[camocorvette]

All the heavy barrel guns I've attached it to I've had no issue. Dasher hits a tenth low at 4,8 and 1000 with it on. Anschutz 22 no change in poi. Ar15's haven't noticed a change in poi.

 

[macandrac]

All the heavy barrel guns I've attached it to I've had no issue. Dasher hits a tenth low at 4,8 and 1000 with it on. Anschutz 22 no change in poi. Ar15's haven't noticed a change in poi.

Thanks for the feedback, I'm not sure if the PSA PA-65 is considered a heavy barrel but the gun weighs in at 14.5 lbs with 20 rounds loaded so I'm assuming it is.

 

[macandrac]

All the heavy barrel guns I've attached it to I've had no issue. Dasher hits a tenth low at 4,8 and 1000 with it on. Anschutz 22 no change in poi. Ar15's haven't noticed a change in poi.

IMO humble opinion and experience your "accuracy issue" is likely due to adding weight to the end of the barrel thus changing harmonics. Your velocity increase could be from a new barrel speeding up(if its new) and/or the gun getter hotter affecting powder stability thus increasing pressure/velocity.

Also, in my experience the magnetospeed has not affected accuracy when running a silencer and/or heavy barrels. Ive shot many PRS stages over the years with a magneto attached. You know, because primetime to confirm your MV is during the match.

Thanks Scott, it just seems like my groups are expanding when I have the MagnetoSpeed Chrono attached. I'll try again to see if this pattern continues.

 

[macandrac]

I would suggest looking into the Wiser mount. It attaches to your chassis and has a rod extending parallel to the barrel where you attach your bayonet thus taking it off the barrel and eliminating the POI shift

I'll look into the Wiser mount and see if that makes a difference - thanks for the feedback.

 

[macandrac]

1) really isn't surprising at all, as @Scott_at_Vortex mentioned, you've modified the barrel's harmonics.

2) How much fps increase are you talking about? What is the round count on the barrel? How warm is the barrel for each of the 20-shot strings? It's August and you're in Florida... is your ammo sitting in the sun while you're shooting the strings?


Real pros challenge themselves maneuvering inside a bus with a 42" barrel with a suppressor & a MagnetoSpeed attached

I haven't been leaving the ammo in direct sun but it has been around 90 degrees when I've been shooting. I've probably only waited about 5 minutes between 20 round shot groups so barrel doesn't cool in between but does drop some in temp. The average Velocity between the 1st group of 20 rounds and the last group of 20 rounds increased by 39 fps.

 

[R_A_W]

90deg isn’t anything to worry about, I t’s not the ambient air temp

5 minute pause between 20 round strings isn’t very long of a cool down, so your chamber will still be pretty warm & then heat right back up within the first few rounds.

If you’re using a powder that is very sensitive to temperature and you are chambering a round into a hot chamber and then sitting for 60 seconds to take that shot, you would see more than a 39fps increase

I’d give the barrel more time to cool. It may be a heavy barrel for an AR, but it’s pretty light for a 6.5CM

 

[macandrac]

Thanks and that makes a lot of sense - I've been running Sellier & Bellot 140 Gr FMJ mostly since economical to shoot.

 

[R_A_W]

I don’t know this for sure, so take this with a grain of salt… but I have the feeling that the S&B ammo is 1) hotter than a lot of factory ammo and/or 2) the powder isn’t the most temp stable.

I watched a friend shoot S&B 140gr soft points out of a 6.5 CTR a few weeks ago. I didn’t bring a chrono for him to use, so we were just doping @ range. We had to constantly up the velocity in his ballistics app to get the right dope. Initially, because it was shooting much faster than I expected for factory 140gr 6.5CM (particularly because it was out of a Tikka). Later, because it seemed to continue to speed up, but then I saw he had left his ammo in direct sunlight & it was 105 out (barrel wasn’t that hot), so we paused until his unfired rounds returned to a normal temp (they were actually very warm/hot to the touch).

If S&B is in fact a hotter load and/or uses temp sensitive powder, I would expect more erratic performance towards the end of longer shot strings & on subsequent shot strings as your barrel heats up.

Nothing you’ve said makes me think there is a problem with your gun or the ammo. Just learn the limitations of both & utlize them accordingly.

Cheers

 

[macandrac]

Update - I went back to the same Range 2 days ago (Aug 18th) and shot 10 rounds of S&B 140 Gr FMJ without Chronograph then let barrel cool and shot 10 rounds of S&B 140 Gr FMJ with Chronograph. I had to adjust Scope's Elevation down 2 1/4 MOA (4 1/2" @ 200 yards) once Chronograph was attached in order to achieve same POI. I even changed out the Chronograph spacer (to thicker barrel spacer) thinking my bullet may be traveling a bit close to Chronograph but that didn't change anything. I'm so baffled it hurts - I can understand a small change in POI but not 2 1/4 MOA. Note: I cinched the Chronograph down pretty tight to ensure it wasn't moving between shots.

 

[spife7980]

I'm so baffled it hurts - I can understand a small change in POI but not 2 1/4 MOA. .

Don’t be, that’s just what hanging weight on the end of your free floated barrel does.

 

[Chris1972]

I think you’ve just changed the harmonics of your barrel with it attached. Simple as that.

 

[Tyler Kemp]

If you'd like more robustness and adjustability, I've sold thousands of my "Magneto Mounts", and offer a discount to hide members:

MK Machining Gen III Magnetospeed Mount

MK Machining Gen III Magnetospeed Mount We’ve made substantial improvements to our very popular Magnetospeed Mount for better performance with long barrels and suppressors. Beefier construction, larger hardware, Unique 3-piece clamp works with both Arca & Picatinny rails. Fully CNC machined 6061

www.mkmachining.com

As mentioned above, you've attached a weight to your barrel, altering the harmonics of it. You will absolutely have potential POI and accuracy shift, and the mount of your choice will pay for itself in ammunition gathering more data/practice rather than solely velocity.

 

[Gohring65]

Here’s a small sample experiment with my 20” AR 15 and 75 grain bthp at 200 yards. Left is no magnetto attached, right is magnetto attached. Not much difference, not enough that I’d probably go ahead and leave the magnetto on if I was to work up a load on that particular rifle.

 

[Aftermath]

Don't leave the chrono attached if you are looking for your best accuracy. Use the chrono to collect data and disregard where those shots impact. In fact, I usually just shoot at the berm if the thing is hanging on the barrel.

 

[macandrac]

Don’t be, that’s just what hanging weight on the end of your free floated barrel d

Don’t be, that’s just what hanging weight on the end of your free floated barrel does.

Thanks for the feedback, what I would have expected is POI to be lower with the weight attached but it is higher with the Chrono attached.

 

[Chris1972]

It’s not the weight it’s the time that changed.