Do mild loads result in higher POI?...


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KBintheSLC
October 6, 2010, 10:16 PM
I bet this has already been discussed here, but the search didn't yield much for me.

I recently loaded up some very light .40 S&W loads to shoot out of my Glock 20 conversion. The fixed combat sights are dead on when using full power 10mm Auto loads out to 25 yards, but the point of impact is 5" high at 15 yards when I switch to the light .40 loads. Is this the result of ballistics, or is it possible that the Lone Wolf conversion barrel could alter POI that much?

Thanks, KB

PS... the .40's are a "starting load" 165g plated bullet at about 975 fps.

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Clarence
October 6, 2010, 10:21 PM
When shooting a handgun light loads will typically print higher than full power loads.

The reason for this is that when using full power loads the bullet will exit the barrel earlier during the recoil cycle......in other words as the barrel is rising as a result of the recoil, the full power load exits earlier than the mild load, hence the lower point of impact.

KBintheSLC
October 6, 2010, 10:24 PM
Interesting. I wonder if I might have better results by adding another 50 fps. It's too bad because the light load is grouping very nicely and is a real pleasure to shoot.

bds
October 6, 2010, 11:40 PM
Try shooting several test loads at higher charges and see if the POI drops.

KBintheSLC
October 7, 2010, 01:17 AM
Try shooting several test loads at higher charges and see if the POI drops.
Absolutely. Its a new caliber for me, so I am still trying to work up a functional light load. I have a few new ones that step up about 0.1 grain at a time, but don't have time to get out and test them soon enough.

Tilos
October 7, 2010, 11:11 AM
Slower bullet speed, equals longer barrel time during recoil, equals higher point of impact.

Try a "faster" powder near the max end of load data.

Also, Glock makes 3 different height rear sights at about $5 each and a cheap/cheesy adjustable.

1SOW
October 7, 2010, 11:06 PM
Absolutely. Its a new caliber for me, so I am still trying to work up a functional light load. I have a few new ones that step up about 0.1 grain at a time, but don't have time to get out and test them soon enough.
Usually .1 grains of change will give little to no change. Try .2gr jumps for test loads.

FWIW: Have you got access (own, borrow, rent) to a chronograph? Sometimes the bullet speed can be surprisingly different from what you thought it might be by 'feel'. When you get the bullet speed and felt recoil you want, then consider sights to fit your favorite load.

jbrown13
October 8, 2010, 09:37 AM
I'm not sure there is a blanket statement that can be made that applies in all cases. My experience indicates that with a Ruger 357/38 revolver that 357 magnum loads print higher than 38 Special loads. There have been previous discussions about this on THR. As an example here is one which indicates that full power revolver loads will print higher than mild revolver loads.

http://thehighroad.org/showthread.php?t=539537&highlight=POI

If there is a proven reason why this is not the case in a Glock, I would appreciate the source of the data that proves it.

Walkalong
October 8, 2010, 10:14 AM
Recoil affects POI, as well as barrel dwell time. It is not always cut and dried, but usually the somewhat lighter load will print higher. A drastically different load is another issue, but what the OP is asking about is a somewhat lighter load than full power in the same caliber, so the answers he received are correct. Yes, that is why they are impacting higher.

Jim Watson
October 8, 2010, 10:16 AM
I think that much difference is largely due to barrel fit, less so to dwell time and recoil.

Walkalong
October 8, 2010, 10:18 AM
How so Jim, I am curious how that affects it?

jbrown13
October 8, 2010, 11:59 AM
Recoil affects POI, as well as barrel dwell time. It is not always cut and dried, but usually the somewhat lighter load will print higher. A drastically different load is another issue, but what the OP is asking about is a somewhat lighter load than full power in the same caliber, so the answers he received are correct. Yes, that is why they are impacting higher.
Walkalong, admittedly the pressure differential between a light 40 S&W load and a max load is relatively small compared to the delta P between 38 Special and 357 magnum loads. However, I still can't get my head around barrel dwell time being unequivocally the reason for 5" higher POI. The barrel dwell time difference has got to be in the hundredths, maybe thousandths, of a millisecond at that delta P, and for that little time to create a 5" difference in POI is hard to comprehend. Add to that all the unknowns from the OPs post (powder charges, powder brand, OAL, Ransom Rest, Sandbags, freehand, etc.) I just can't accept that barrel dwell time is the unequivocal factor impacting the 5" difference. If my rough calculations are correct that is around 32 MOA.

I will agree that barrel dwell time is a contributing factor in POI, but at some point recoil (pressure) differential cancels dwell time and becomes the major factor in POI. I don't think any of us can be absolutely certain where that point is.

Walkalong
October 8, 2010, 12:06 PM
I still can't get my head around barrel dwell time being unequivocally the reason for 5" higher POI
5" is a lot, for sure.

I don't think any of us can be absolutely certain where that point is
I agree.

rcmodel
October 8, 2010, 12:20 PM
The simple test of whether it is dwell time or an after-market conversion barrel fit problem is to shoot some full power .40 S&W in it and see where it hits.

If the groups come down, it is low power dwell time.

If they stay in the same place, it is a conversion barrel fit problem.
In otherwords, it isn't locking up in the slide in the same place the 10mm factory barrel locks in place.

rc

jbrown13
October 8, 2010, 12:56 PM
RC, you're right as usual. And shoot the test loads out of a Ransom Rest to take the human element out of the results as much as possible.

KBintheSLC
October 8, 2010, 02:09 PM
Some good feedback so far. I too am suspicious as to whether this could be caused entirely by dwell time. It may be the aftermarket barrel.


FWIW: Have you got access (own, borrow, rent) to a chronograph? Sometimes the bullet speed can be surprisingly different from what you thought it might be by 'feel'. When you get the bullet speed and felt recoil you want, then consider sights to fit your favorite load.

Yes. The velocity stated in the OP was an average derived from actual chrono testing that I performed. I will step up the load a bit to see if it fixes things. If not, I may have to swap out the fixed rear night sight for an adjustable one.

I am hoping to keep the load as light as I can for falling plates matches.

Tilos
October 8, 2010, 02:34 PM
KBintheSLC:
Yes, kind of what I said way back in post#6.

I had asked before, for advise about changing a load to lower the point of impact.
I mostly got critics and no usable info, and was even called "silly".

I switched to Bullseye (a faster powder) and was able to get poi to point of aim, at a low velocity.

Edited to add:
If this high POI is related to barrel lockup, I don't see Lone Wolf remachining it, and the fix is still the same....different load, different sight, or both.

Walkalong
October 8, 2010, 02:42 PM
Silly me, I saw "conversion barrel", but it didn't register. :o

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