Seating depth and velocity

[Oldly]

Last week I installed a mtd chassis on a savage 10t,this allowed me to load longer cartridges,I also orderd from a hornady gauge.so loaded 69gr sierra .020 off the lands and then loaded 5 with 24.5 gr and 5 24.8 and 5 25.1 with varget.went to the range yesterday and tested them,the 24.5 had velocity of 2960 and es of 7 the 24.8 were around 3000 and es was 12,I never bothered to try the 25.1 because of the results I was getting from the first 2 so why load .previously I was losing 25.1 at 2.25 mag length and was not getting these velocity or es.does seating effect velocity that much.the only problem with the new chassis is the cartridge goes to roof of chamber and then makes the follower collapse.never happend with factory stock are mag,waiting to hear back from mtd.

 

[todbartell]

closer to the lands will have higher pressure / velocity than seating further off the lands. You can get more velocity with less powder like you're experiencing

 

[Maple57]

Closer to the lands does not necessarily correlate to higher pressure. That statement is far too broad to be factual.

Extending your seating depth increases your case capacity and that lowers pressures with a given powder charge.

Too much jump typically correlates to wide velocity spreads and poor accuracy. As seating depth is increased, the velocity spreads tend to tighten up as accuracy improves... to a point.

Eventually things start to go the other way because the bullet is no longer getting a running start into the rifling. That's when velocity spikes tend to occur. It is also dependent upon throat wear and throat angles and may not even occur if you have enough rounds down the pipe. So problems like this may go away all by itself after enough rounds are fired.

If you are having trouble with MDT mags... I assume AI pattern mags, try the Accurate Mag for 223... Bullseye London had them last time I checked but they are branded as Cadex mags. Accurate mags are the only mags I will use for my 223.

Oh ya, they can be easily modified to suit longer seating depths. Check out Practical Tactical on youtube for a video.

 

[mimile]

This is how I understood it but I might be wrong.
Seating the bullet in direct contact to the rifling can increase the pressure significantly (sometimes dangerously) and with it velocity. That's why it is advised to seat bullets 20-30 thousands of an inch away from the rifling. Normally, away from the rifling, increasing seating depth would increase the pressure and with it the speed as the gas created by ignition has less volume to expand. Unless the throat is very long in which case some of the gas with leak to and expand in the throat which in turn will reduce the pressure and velocity.
Briefly, given all that, the only way to know what happens with velocity, accuracy and pressure is to experiment with loads.
Ganderite is the expert on all these matters. He can illuminate us.

 

[Oldly]

Closer to the lands does not nesesarily correlate to higher pressure. That statement is far too broad to be factual.

Extending your seating depth increases your cae capacity and that lowers pressures with a given powder charge.

Too much jump typically correlates to wide velocity spreads and poor accuracy. As seating depth is increased, the velocity spreads tend to tighten up as accuracy improves... to a point.

Eventually things start to go the other way because the bullet is no longer getting a running start into the rifling. That's when velocity spikes tend to occur. It is also dependent upon throat wear and throat angles and may not even occur if you have enough rounds down the pipe. So problems like this may go away all by itself after enough rounds are fired.

If you are having trouble with MDT mags... I assume AI pattern mags, try the Accurate Mag for 223... Bullseye London had them last time I checked but they are branded as Cadex mags. Accurate mags are the only mags I will use for my 223.

Oh ya, they can be easily modified to suit longer seating depths. Check out Practical Tactical on youtube for a video.

I'm not sure you can use any other mag with the mtd chassis except there's. Mtd told me my baffle is to long and that needs to be resolved,I just don't understand how it can't be the mag but that issue I posted in the general forum.but seating .020 off the lands gave me an es I have never achieved at loading 2 standard mag length before.it is going to be interesting fidgeting with seating depths now that I can load beyond standard mag length.

 

[todbartell]

Closer to the lands does not necessarily correlate to higher pressure. That statement is far too broad to be factual.

Extending your seating depth increases your case capacity and that lowers pressures with a given powder charge.

6mm Creedmoor, 24" 7.75t Benchmark 1:8t
105gr Berger VLD Hunting
Alpha brass, Fed 210M primer, Winchester StaBALL 6.5 powder

velocity measured with Labradar

Jam lands 3116 FPS

.010 jump 3104 FPS

.040” jump 3092 FPS

.080” jump 3078 FPS

.120” jump 3050 FPS

 

[Maple57]

6mm Creedmoor, 24" 7.75t Benchmark 1:8t
105gr Berger VLD Hunting
Alpha brass, Fed 210M primer, Winchester StaBALL 6.5 powder

velocity measured with Labradar

Jam lands 3116 FPS

.010 jump 3104 FPS

.040” jump 3092 FPS

.080” jump 3078 FPS

.120” jump 3050 FPS

Ya ya

read my whole post.

 

[LittleBoyLefty]

 

[Oldly]

View attachment 496872

Thats some change in pressure.

 

[todbartell]

Ya ya lol

 

[Maple57]

Ya ya lol

English must be your second language huh.

 

[todbartell]

6mm Creedmoor, 24" 7.75t Benchmark 1:8t
105gr Berger VLD Hunting
Alpha brass, Fed 210M primer, Winchester StaBALL 6.5 powder

velocity measured with Labradar

Jam lands 3116 FPS

.010 jump 3104 FPS

.040” jump 3092 FPS

.080” jump 3078 FPS

.120” jump 3050 FPS

View attachment 496872

Closer to the lands does not necessarily correlate to higher pressure.

 

[bearhunter]

closer to the lands will have higher pressure / velocity than seating further off the lands. You can get more velocity with less powder like you're experiencing

A really good example of what todb is referring to is to compare different rifles with similar chambering.

A custom 7x57 chamber, cut with a reamer that is ground for a specific cartridge length will usually give at least a couple of hundred fps more velocity than a rifle with a chamber cut at the factory.

The reason for this is the long throats. Freebore is a velocity/accuracy killer. Accuracy won't suffer if the throat dimensions are tight but velocities will definitely be affected.