boreline to chassis bottom for positional shooting

[Takashi]

I've been thinking a lot about the distance from the center of the bore to the bottom of arca as i transitioned from an AT to an AT with obsession chassis.
Between these two I've definitely noticed it matters for roll/cant control but it seems that it might be more of a center of gravity thing too, as well as moving the recoil impulse to a more favorable position in the buttpad. I know the new AT-X also emphasized a lowness in the chassis.

I'm looking to build an impact action and theres quite a bit of variation in options--is this something you guys look for? It seems the MDT ACC might be the lowest, and everyone a fair bit higher.

 

[Stoweit]

The new Manners TCS is very low as well, seems like trend.

 

[Average guy]

The lower center of gravity will definitely lend to the rifle being more stable. The atx chassis weights are also of the correct size to make the stock wider at the bottom lending more stability

 

[1moaoff]

I have been begging for lower center chassis forever. I think its a slow process because of visuals and the appearance compared to others.

 

[Takashi]

I think trying to conceive the difference in recoil impulse inside of 1/4" of bore height/ bottom of fore-end is like running a 105 at 2800 instead of 2900 because you think it recoils less.

(Caveat: I'm 6'3", 225lbs and exercise)

In my limited experience from my scopecam vids on a barricade with not a lot of shoulder input (but not free recoil), it can mean the reticle moving off target closer to 3 mils instead of 6 mils or more, which for me makes a delay in tracking the trace of the bullet going into the target. So my 10%ish shoulder loading on the rifle will be more effective at follow through than it otherwise would be, if that makes sense.

 

[samb300]

There are a couple things at play. The forend to bore axis distance definitely helps with stability on props, as it reduces cant and lowers the COG of the rifle closer to what it's sitting on. That makes vertical and cant wobble less. But the balance of the rifle matters a lot too; anything that is rear heavy will magnify the "up" reticle movement during recoil.

R700 chassis' are limited to how close the bottom of the forend can be to the bore axis by the height of the recoil lug. I'm sure the ACC, MPA, Vision, XLR, etc are all pretty close to the same assuming they can all accept an "x" height recoil lug. If the atx-R700 chassis ends up being significantly closer to the bore axis than those, I'm not sure how they'll have done it to accept a wide variety of recoil lugs and maintain forend stiffness.

 

[samb300]

The lower center of gravity will definitely lend to the rifle being more stable. The atx chassis weights are also of the correct size to make the stock wider at the bottom lending more stability

This is the same for pretty much every chassis and external weight out there, the atx does nothing different. External weights add width to the forend, but cannot get any closer to the bottom of the forend without interfering with the ability for the RRS (arca) dovetail accessories to clamp.

 

[Average guy]

The Atx was a good example to use, as it was featured prominently in Frank’s videos. It was a highlighted feature. Figured that would be the easiest reference out for people to get eyes on

 

[Stoweit]

That AO chassis looks great!

 

[Reload10]

Desert tech srs probably has the lowest, as well as the bag being able to be placed super far back.

 

[Takashi]

The SRS feels like it'd be very, very, backheavy with virtually no hope of balancing.

There are a couple things at play. The forend to bore axis distance definitely helps with stability on props, as it reduces cant and lowers the COG of the rifle closer to what it's sitting on. That makes vertical and cant wobble less. But the balance of the rifle matters a lot too; anything that is rear heavy will magnify the "up" reticle movement during recoil.

R700 chassis' are limited to how close the bottom of the forend can be to the bore axis by the height of the recoil lug. I'm sure the ACC, MPA, Vision, XLR, etc are all pretty close to the same assuming they can all accept an "x" height recoil lug. If the atx-R700 chassis ends up being significantly closer to the bore axis than those, I'm not sure how they'll have done it to accept a wide variety of recoil lugs and maintain forend stiffness.

Sam, thanks for this! By this logic it would make more sense to go lighter in barrel contour and heavier on weights if it lowered CoG?
Other than contour for weight, how does one pick a barrel thickness?

 

[Guns&WhiteWater]

Desert tech srs probably has the lowest, as well as the bag being able to be placed super far back.

My MDT ESS is pretty low too. Love that thing.

 

[eamars]

The SRS feels like it'd be very, very, backheavy with virtually no hope of balancing.

Sam, thanks for this! By this logic it would make more sense to go lighter in barrel contour and heavier on weights if it lowered CoG?
Other than contour for weight, how does one pick a barrel thickness?

I'm going to put a 30" straight contour barrel on my SRS A1. Hope this could balance the center of gravity to the end of the handguard.

 

[Reload10]

I'm going to put a 30" straight contour barrel on my SRS A1. Hope this could balance the center of gravity to the end of the handguard.

It will balance extremely well. I have mostly mtu 28" barrels on mine with a can, amd the balance point is ~4" in front of the trigger guard.

 

[samb300]

The SRS feels like it'd be very, very, backheavy with virtually no hope of balancing.

Sam, thanks for this! By this logic it would make more sense to go lighter in barrel contour and heavier on weights if it lowered CoG?
Other than contour for weight, how does one pick a barrel thickness?

Forend weights (internal or external) aren't going to appreciably raise or lower the system COG since they're basically at the same height relative to the bore axis.

I think the optimal solution is to go with as thick/heavy of a barrel as you can, and add weight to tune the balance to your liking. Not only is weight in the barrel free since they all start as 1.25" dia blanks, but then you can get a well balancing rifle without needing to add a bunch of shit to it to balance.

I made a COG calculator spreadsheet since I have almost everything for the AI system in CAD, and figured out exactly what barrel contour I would need to balance ~3" in front of the magwell. Turns out for me it's a 24" MTU that finishes at 1.00" at the muzzle, with an Ultra7 on the end, and the 2# internal weight in the chassis. No external weights needed unless I just want to add more weight.

 

[308pirate]

Forend weights (internal or external) aren't going to appreciably raise or lower the system COG since they're basically at the same height relative to the bore axis.

The question (paraphrased) was "will the CG be lower if the barrel is made thinner and a compensating mass added below it with chassis weights"?

The answer is yes

 

[samb300]

The question (paraphrased) was "will the CG be lower if the barrel is made thinner and a compensating mass added below it with chassis weights"?

The answer is yes

Well...yes. But you need to ask, do you want the COG to be lower or higher?

What does bringing the forend closer to the bore axis do, does it lower or raise the COG of the rifle?

 

[308pirate]

Well...yes. But you need to ask, do you want the COG to be lower or higher?

I don't care one way or the other. I'm a firm believer, based on my competition experience in NRA Highpower and USPSA, that equipment and firearm nuances matter a whole lot less than most think they do.

What does bringing the forend closer to the bore axis do, does it lower or raise the COG of the rifle?

That depends on what you move, the bore axis or the forend, in relation to the other masses in the rifle system

 

[brianf]

when i was shooting my 300wm much more i had a QD scope mount id attach

turned down a steel shaft at work from old piece of equipment that i mounted in the scope rings

would attach it to the bottom rail right below the mag well (wasn't using barricades etc)

would "anchor" the rifle and add a few pounds for recoil

also had a trg-42 in 338lm...that thing felt so top heavy it wanted to flop over...at lease to me