Shooting off a bench question

[ELKOHOLIC]

I have got a bench I use when sighting in my guns. It has an arm that extends upward and cradles the end of the barrel. This arm is covered with a rubber material. I also use a sand bag that the gun sets in. Last year I noticed a big problem when shooting my Browning A-Bolt 7mm Rem Mag. At first I thought my scope was off, but I figured out what it was.

I wasn't always using that arm to rest the barrel in when shooting. When I did, there was a dramatic difference in point of impact. When using the rest for the barrell my bullets would impact 10" higher than normal.

I have since done away with using that and I just shoot with the sandbag that doesn't touch the barrel. I decided to try something this year though and I tried putting a bungy chord over the barrel when using the arm and it still impacted way high.

Anyway, any idea what is causing this? Obviously the barrel is jumping up before the bullet leaves the barrel, but has anyone else noticed this before.

 

[cjg_beef]

Anything touching the barrel will affect the point of impact. That is the reason why people free float their barrels. It makes it so that nothing is touching the barrel. Even your hand touching the barrel can affect the POI.

 

[ELKOHOLIC]

I see what your saying, but they make the rest on the bench for that purpose. Is that normal for a gun to impact that much higher at 100 yards with the barrel resting on something?

 

[Chef]

I don't know the scientific answer to your question.
I never rest the barrel on anything. I prefer to rest the forearm of the stock on the front cradle or bags.

This is a good pointer/reminder though for those that would rest their barrel on a branch or stump whilst out in the field.

Good post.

 

[cjg_beef]

>I see what your saying, but
>they make the rest on
>the bench for that purpose.
>Is that normal for a
>gun to impact that much
>higher at 100 yards with
>the barrel resting on something?
>

Is this the one you have?

http://www.sportsmansguide.com/net/cb/cb.aspx?a=334743&kwtid=236951#

If so the designers of the bench have it all wrong. It looks like you might be able to rotate the front rest around so that it's resting on the forearm like the link below...

http://www.cabelas.com/cabelas/en/t...340306&cm_ite=0016127227876a&_requestid=37350

 

[sageadvice]

ELKOHOLIC, Yeah, these guys are right. Resting your barrel on anything is a very bad idea. I also believe that your barrel is jumping up off the soild rest before the bullet leaves the barrel. I kinda figured this out on my own, many years ago but for whatever the possible reason, have just avoided having my barrel touch anything while shooting. My understanding is that all barrels have a wave like effect when fired but i best leave that detail and effect to those with more knowledge than i.

I'm no super bench rest specialist but do shoot with guys that are and i'm often surprised with the very minor variations in pressure to the forearm, stock, trigger, cheek, or shoulder, that will cause point of impact on the target to move. The trick is to find out how your gun likes to be shot and then do the exact same thing, down to the very smallest detail, time after time. Makes little holes!

Joey

 

[Pre64]

A couple other things that will affect your POI;

Are you using a hard surface to rest your stock on? Rifles will "jump" from a hard surface as opposed to sand bags. Also, mind the position of the swivel slug on the forestock. If it is just in front of or on the sandbags it can also make the rifle shoot high. There's lots of other reasons from scope problems to hot barrels to improper rests on both ends of rifle to a burred crown......etc.

 

[RELH]

the direct cause of your rifle shooting high is the fact the barrel is touching the rest instead of the forearm. When a object touches the barrel, the shot be off.
If the object was touching the barrel on the left side, the shot will go to the right, bottom side shot will go up as in your case.
You need to fix that rest where the rife forearm will rest on the front bag and not the barrel.

RELH

 

[ELKOHOLIC]

cjg_beef, Thats the one I have. The rest can be turned inward and probably be used on the stock. Whats funny is in the link it shows the guy using it just like I used to. I never would have guessed it would of made that big of a difference. Now I know better!

 

[FullCurlHunter]

A more scientific answer...

There are a lot of accuracy buffs that discuss barrel harmonics. Basically, when you shoot your barrel creates a harmonic wave as the bullet is heading down the rifling, leaving the barrel and immediately after. The best accuracy comes at a perfect combination of bullet, powder, form where the bullet leaves the barrel when the wave is level or neutral. This is where the shots are repeatable and group sizes are smallest. More often than not a quality bullet will shoot in a rifle simply by varying the amount of powder by a small amount. This creates a different wave. I wont go any further as I am not sure if you reload or not...

Sometimes guns come with a pressure point applied to the tip of the stock that touches the barrel. More often than not, the point of impact will change with varying temperatures/weather conditions especially with wood stocks when it has a pressure point. The reason for the point of impact changes are that the stock can contract and expand with wet/dry/hot/cold conditions creating a different amount of pressure on the barrel ultimately affecting the harmonics of the barrel and point of impact.

This is why so many swear by free floating sporter and heavy barrel rifles... some lightweight contour barrels actually benefit from a pressure point. (ore often than not, the stiffness that the pressure point provides on these light barrels outweighs the negative affects of the pressure point by weather especially with a composite stock).

So, in a sense this is what you have seeen, resting your bare barrel on somethign while shooting is creating pressure and changing the harmonics of the barrel. You noticed a 10" difference when not doing so and using a sand back on the end of the stock.

I have witnessed a 4" change on a few of my rifles simply by adding bipods to the front swivel versus shooting off of a sand bag. A solid recoil absorption with the sand bag versus a dramatic jump off the rest with anything hard like bipods or the plastic barrel rest in your case...

Ultimately, we are after repeatable point of impact under field conditions for hunting.

I would immediately stop resting the barrel on the rubber rest at the very minimum. (unless you are goign to carry the bench hunting!)

I also think that it is rather strange that a shooting company would produce such a product. Maybe they don't knwo any better. Why would anyone want to buy their product if it encourages poor accuracy...

Those are my quick thoughts.