Higher Powder Charge, Lower FPS

[gelton]

Just thought I would post this here as I am in the process of working up a load for my .300 WM and had never seen this before so I thought I would share.

Gun - Weatherby BackCountry .300 WM 26" barrel
Powder - H1000 (Also have some IMR 4350 that I may tinker with but Nosler says this is the most accurate for 180gr Accubond)
Bullet - 180 gr Nosler Accubond
Charge 82.5 Grain (this is over max so do not start here) = 3003 FPS with a phenomenal ES (just a 3 shot group though)
Charge 83 Grain (again over max) = 2975 FPS

This intrigued me as I have never seen this before...obviously with 83 grains of powder the case is full as can be (all loads are compressed) but I never would have guessed a higher charge would = lower velocity.

FWIW the 82.5 charge is the best group I have ever shot out of this gun and was about 1/2" MOA...5/8 to be exact. I was hoping for 3100 FPS and may try some IMR 4350, I should be able to get there fairly easily with plenty of room left in the case, however, I haven't even started messing with seating depth testing and am pretty darn happy with 5/8" MOA out of a factory gun.

After doing some research it sounds like this (slower velocity with higher powder charge) isnt as uncommon as some might think...any input or similar experiences?

 

[Mule3006Elk]

When you see velocity drop with increasing powder charge you're very likely over pressure.

 

[Mule3006Elk]

BUT....a lot of rifle shoot really well on that threshold. Not an endorsement just saying.
My kids shoot my rifle so I tend to back off when I see chrono data or objective data that suggest I'm over pressure.

 

[gelton]

BUT....a lot of rifle shoot really well on that threshold.

Funny thing is there are no pressure signs, bolt lift is normal etc...but the case is just way too full at 83 grains for any sort of consistency IMO. On a side note, I was shooting some Swift Scirrocco II's 180 grain factory ammo and had a couple of blown primers and pressure signs which I thought was crazy but I was getting the 3100 fps I was chasing.

 

[Mule3006Elk]

Factory can be crazy. I was shooting Federal Edge TLR out of my 300 WM. Velocity is suppose to be 2800 fps or so. I was getting close to 3,000 fps. Something was not right. Chronograph with magnetospeed.

 

[brn2hnt]

I’m always suspicious of chronos.

But I would have expected a bit more velocity with the 180’s?

I’m getting 3030 (calculated by drops) with 215’s out of a 26” tube.


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[gelton]

I’m always suspicious of chronos.

But I would have expected a bit more velocity with the 180’s?

I’m getting 3030 (calculated by drops) with 215’s out of a 26” tube.


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I would be suspicious if I hadn't seen higher velocities out of factory bullets and the same chrono...I haven't been able to verify it at long range though (shooting on the back part of my property) but I am a bit disappointed that the Nosler load data shows 3123 fps with 81 grains of H1000 and I am only getting 3000 with 82.5 but the accuracy is there.

FWIW - this gun has always been sub par compared to others from a velocity standpoint. I was using 215 Berger Hybrids and was getting 2800 (confirmed at range) while others like you were getting close ~3K. And this is with 77 grains of H1000 which was way over max and beginning to show pressure signs.

 

[brn2hnt]

I would be suspicious if I hadn't seen higher velocities out of factory bullets and the same chrono...I haven't been able to verify it at long range though (shooting on the back part of my property) but I am a bit disappointed that the Nosler load data shows 3123 fps with 81 grains of H1000 and I am only getting 3000 with 82.5 but the accuracy is there.

FWIW - this gun has always been sub par compared to others from a velocity standpoint. I was using 215 Berger Hybrids and was getting 2800 (confirmed at range) while others like you were getting close ~3K. And this is with 77 grains of H1000 which was way over max and beginning to show pressure signs.

Gotcha. I’ve got a custom throated chamber for 215’s and only running 76.5, so that likely helps a bit.

Sounds like you’ve definitely got a slow barrel. It is what it is, just have to work with the barrel you’ve got, and accuracy trumps all anyways.


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[Mule3006Elk]

accuracy trumps all anyways.

This!!
Animal isn't going to know the different between 100 fps +/-. You are already shooting a 300 WM. Plenty of velocity. BUT...I get it you want to maximize the ballistics. You might have a slower barrel but if it's accurate I wouldn't care.

 

[JakeSCH]

What brass are you using? My ADG brass is slightly compressed at 77 grs of H1000.

I've seen velocity drops right when I exit a node because the ES opens way up. Was the 2975 a 3 shot group or a single shot?

At the end of the day 3000 with a low ES and 1/2 MOA is money.

It also might be worth checking the 181 gr Hammer Hunters (what I am shooting), it has a higher BC than the accubond and with the lower bearing surface you might get more velocity (if you really want it).

 

[gelton]

What brass are you using? My ADG brass is slightly compressed at 77 grs of H1000.

I've seen velocity drops right when I exit a node because the ES opens way up. Was the 2975 a 3 shot group or a single shot?

At the end of the day 3000 with a low ES and 1/2 MOA is money.

It also might be worth checking the 181 gr Hammer Hunters (what I am shooting), it has a higher BC than the accubond and with the lower bearing surface you might get more velocity (if you really want it).

Nosler brass...the 2975 was a 3 shot average.

 

[rayporter]

shoot it again and average 6 shots for each load. sometimes you dont have enough data to make an informed decision. barrel temps will cause this and the first shot of the day in the first string can be off. foul the barrel with a shot and then drop that shot from your data.

 

[gelton]

shoot it again and average 6 shots for each load. sometimes you dont have enough data to make an informed decision. barrel temps will cause this and the first shot of the day in the first string can be off. foul the barrel with a shot and then drop that shot from your data.

This is what I plan on doing except for seating depth not powder charge. I just loaded 15 rounds, 5 for each seating depth....for me 5/8" @ 100 with a factory rifle is a happy place but I will see if I can improve on that with 3 different seating depths. The original and then 10 thousands longer for the other two.

If it gets better than I am happy as a pig in shit, if it gets worse I will most likely do more powder testing and may try some IMR 4350...if it stays the same, I am happy still.

 

[swampthing]

I recently saw this on a 7 rem with the higher charge, lower velocoity. The guy thought maybe he mixed up his loads but a second batch of loads showed the same results! There was no pressure signs here as well. Who woulda thought!!

 

[CAhunter]

I experienced the same thing recently working up a load in my 6 Creed. I saw a drop of 20 FPS over six shots going from 44 gr H4350 to 44.4 gr H4350. My seating depth though with my higher charge was seated out .01” closer to the lands, comparing it to a prior load. I didn’t see any pressure signs either. I haven’t been back to the range yet to gather more data and look more in to it but it confused me

 

[gelton]

I experienced the same thing recently working up a load in my 6 Creed. I saw a drop of 20 FPS over six shots going from 44 gr H4350 to 44.4 gr H4350. My seating depth though with my higher charge was seated out .01” closer to the lands, comparing it to a prior load. I didn’t see any pressure signs either. I haven’t been back to the range yet to gather more data and look more in to it but it confused me

yeah from my research there are all kinds of different explanations of why/how this may happen but the general consensus is once you get past a certain point it starts to plateau and lessen with some theories pointing to the powder burn rate and the barrel not being able to burn it all.

 

[Marble]

Two things...

The sciroocos i have noticed do not tolerate as much pressure compared to the accubonds. I wonder if the copper jacket is thicker/ harder. I shoot them out of my 7mm and they shoot great and stay together. Killed tons of animals with them.

I use H4350 at I think 71 grains with the 180 accubonds. Browning a bolt 2, 26" barrel. Kicks it right at 3200. It shoots 1/2" groups and just crushes elk and mulies.

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