ARCA rail bipod placement

[MarinePMI]

Listened to the latest podcast with Frank and Mark, and the whole "having to adjust the bipod on the ARCA rail" piqued my interest. I never put it together until that, but I also have seen some weird accuracy issues at distant ranges since using an extended aluminum ARCA rail on my AT. Going to have to pull that bipod back a few inches (so it's under the front of the fore end) and see if that resolves the issue. I never really thought about it until Frank mentioned it in the podcast, and then it went "click", followed by a low mumbled "sonnofbitch"....

Thanks for that little nugget Frank; it's things like this that keeps me listening.

 

[M113A3]

For those of us that have not heard the podcast can you give a recap?

 

[MarinePMI]

Essentially, those extended aluminum ARCA/RRS rails tend to flex some. This leads to some vertical stringing at distance, and I'd imagine some horizontal drift if the bipod legs are not equally loaded. At any rate, Frank mentioned seeing it with one of his class students, and the student mentioned just putting the ARCA rail their rifle. Frank and Mark had him move the bipod back, under the fore end, and the vertical dispersion went away. Their anecdotal conclusion was that the rail was flexing ever so slightly, that while not noticeable at closer targets, was clearly demonstrable at the 1000yd target. Makes absolute sense to me...

 

[M113A3]

Thanks! I am using a spigot mount on a KRG W3 chassis so I think I am ok... I only use the spigot in the prone
.

 

[IronOperator87]

I am the student from Alaska prc with the rail that was flexing. I was getting really good groups previously with an atlas psr in a spigot Mount and noticed vertical stringing after I put the rrs spec’d Rail on it with a cal. The cal is a great bi pod but I noticed the rail was flexing up and down when the bipod was all the way out front. You could see it when you pushed on the butt pad of the rifle. I ran it by Frank and he agreed then suggested I move it back and that worked. Groups tightened back up to where they where. Another thing to think about is the spigot interface doesn’t allow a rrs/arca rail to be bolted to the chassis until 1.75 inches back from the front of the chassis so that allows more flex. So you basically have 4” of rail that’s not bolted down sticking out the front on an AT. That 2” of over hang might not be an issue on other rifles.

 

[Subwrx300]

Found the same issue even on Manners PRS2 that has rail that extends forward of stock. I make sure the front edge of RRS clamp is placed directly under a mounting screw for this reason. Had 1-2" extra vertical that I couldn't explain until move bipod rearward to a better supported area of rail.

Glad a few can confirm it wasn't just me!

 

[samb300]

For you guys running an AT, I can understand the issue that's going on. @IronOperator87 you make a great observation about the amount of rail forward of the attachment screw contributing to the amount of flex.

If you haven't seen it yet, check out the bipod spigots I designed and partnered with Anarchy Outdoors to make. I have a picatinny version and a RRS/Arca version. They replace the entire factory bipod spigot block that is housed inside the forend tube, and are rock solid. The RRS version gets you much closer to the bore axis as well. It is designed and engineered to be stiffer than a factory MPA or KRG spigot mount on those respective chassis systems.

The Anarchy page is HERE

The SH post is HERE

Thanks,

Sam

 

[IronOperator87]

@samb300 I like how close the RRS version gets you to the barrel. Looks like it will put the Atlas cal in a position where the apex will be above the barrel. Great stuff

 

[samb300]

@samb300 I like how close the RRS version gets you to the barrel. Looks like it will put the Atlas cal in a position where the apex will be above the barrel. Great stuff

Thanks! Yes, the RRS/Arca version is approximately 1” closer to the bore than the typical forend rail, helping move that apex point up.

 

[Steel head]

Essentially, those extended aluminum ARCA/RRS rails tend to flex some. This leads to some vertical stringing at distance, and I'd imagine some horizontal drift if the bipod legs are not equally loaded. At any rate, Frank mentioned seeing it with one of his class students, and the student mentioned just putting the ARCA rail their rifle. Frank and Mark had him move the bipod back, under the fore end, and the vertical dispersion went away. Their anecdotal conclusion was that the rail was flexing ever so slightly, that while not noticeable at closer targets, was clearly demonstrable at the 1000yd target. Makes absolute sense to me...

Interesting subject.
I felt I had more vertical than usual my last two trips to my 1000+ spots.

I had just starting hanging the bipod out past the forend on a ARCA rail.

I’ll pull my bipod a bit back and try it out tomorrow.

 

[Steel head]

Ok

I’m almost positive I noticed a difference.

Winds were a bit tricky today and lots of mirage yet vertical just wasn’t an issue today as long as I did my part.

 

[Simia Dei]

Found the same issue even on Manners PRS2 that has rail that extends forward of stock. I make sure the front edge of RRS clamp is placed directly under a mounting screw for this reason. Had 1-2" extra vertical that I couldn't explain until move bipod rearward to a better supported area of rail.

Glad a few can confirm it wasn't just me!

I had the same issue, same stock too. Had inconsistent groupings, I may just be shit though. Here's my home bodge solution:

 

[Aaron47pb]

For you guys running an AT, I can understand the issue that's going on. @IronOperator87 you make a great observation about the amount of rail forward of the attachment screw contributing to the amount of flex.

If you haven't seen it yet, check out the bipod spigots I designed and partnered with Anarchy Outdoors to make. I have a picatinny version and a RRS/Arca version. They replace the entire factory bipod spigot block that is housed inside the forend tube, and are rock solid. The RRS version gets you much closer to the bore axis as well. It is designed and engineered to be stiffer than a factory MPA or KRG spigot mount on those respective chassis systems.

The Anarchy page is HERE

The SH post is HERE

Thanks,

Sam

View attachment 7094340

So how much stiffer will these be than the Sawtooth rials? I’m guessing these won’t exhibit the issues described here?

 

[samb300]

So how much stiffer will these be than the Sawtooth rials? I’m guessing these won’t exhibit the issues described here?

I can't comment specifically on the amount of flex exhibited on the Sawtooth or HRD rails when the bipod is placed in front of the forend. I don't have one to test and I'm going off of the experiences of others like the OP when they noticed errant shots due to what they are interpreting as flex.

What I can say is the AI spigot mounts, and the RRS/Arca version specifically, were designed to be stiff enough that the user wouldn't notice or feel flex while behind the rifle and loading the bipod. As I mentioned above, I did extensive FEA during the design process which included reverse-engineering the MPA and KRG spigots. I have never heard anyone comment that they felt flex in either of those mounts, and the mounts I designed are as stiff or stiffer than those two. The picatinny version is significantly stiffer.

The initial feedback I'm getting is positive, with people commenting that they feel really solid under load.

Hope that helps, let me know if you have any other questions!

Sam

 

[IronOperator87]

I am loving samb300’s picatinny version. I recently purchased it after spending over 200$ on 2 other set ups that put the bipod forward of the hand guard on my AT and his is by far the most solid.

 

[Lowlight]

Amazing when you talk about it and make stuff public how many others have "Seen" the same issue but hardly anyone talks about it

 

[IronOperator87]

I know reading this thread would’ve saved me some coin.

 

[FishDr]

Amazing when you talk about it and make stuff public how many others have "Seen" the same issue but hardly anyone talks about it

Interesting.... I have "seen" some vertical stringing with my rifle, even observable at 400-500 yards. Not terrible, but observable. I have been chasing many many things to see if I could resolve the issue. Even chasing tweaking a load. This thread makes sense, since I've been shooting with an extended rail. Gonna check it out this weekend...... I sure hope this is the issue I have "seen"...

 

[Steel head]

Interesting.... I have "seen" some vertical stringing with my rifle, even observable at 400-500 yards. Not terrible, but observable. I have been chasing many many things to see if I could resolve the issue. Even chasing tweaking a load. This thread makes sense, since I've been shooting with an extended rail. Gonna check it out this weekend...... I sure hope this is the issue I have "seen"...

Yea.
I was looking into my load to see if something changed when this thread popped up.

 

[Saluki68]

So simple, such a bone head mistake. Thanks for sharing, I was really getting frustrated. Tightened things right up.

 

[NoLegs24]

So the issue only arose on arca rails that extended past the stock/ chassis?

 

[blbennett1288]

So the issue only arose on arca rails that extended past the stock/ chassis?

Sounds like there were some that were under the chassis, but forward of the RRS mounting points.

 

[IronOperator87]

So the issue only arose on arca rails that extended past the stock/ chassis?

Correct. In my experience the rrs/ arca spec rails need to be bolted down to the chassis With little to no overhang past a mounting screw. The further you put the bipod out past a mounting screw on a rrs rail the more flex you will get, it’s only a thin piece of aluminum. Of course how much you load the bipod will determine how much flex you get if you are mounted out past a fastener.

 

[Saluki68]

I've got ARCA plates on my AR's (both key-mod and m-lock...both with more anchoring points), the short (RRS plate) and full length J Allen rails (very stiff, also geometrically different interface) and have never had noticeable issue with these, including off of a tripod. Its only the "universal" ones on my Manners' stocks that I've struggled with, and after the suggestions in this thread, the issue cleared right up. I wouldn't look at it as a design flaw, but certainly an issue to be aware of. Again, thanks for this post. Huge relief.