Home Forums Hunting & Shooting Long Range Hunting Twist rate for 7 rem with 180 Scenars?

I'm going to re-barrel a 7 rem, what would be the best twist for the 180's?

1-9

Or 1-8 if you'll think you'll want to try the 195 Berger too.

Son's 1-9 twist is shooting 180s very tight. It also shot 195 Bergers well. It's a Rock barrel. I'd go 8.5-9 twist. At TN elevation, maybe 8-8.5 actually.

My 8" twist 7mm-08AI shoots 150 Scenars in to teeny tiny little clusters. Can't see the downside to a little extra twist. I would go 8".

David

I can't find the scenar's length, but the 7mm berger 180 hybrids and VLD's have a similar BC to the scenar and they require an 8 twist according to berger. I'd not go any slower than 1-8 if I were building a rifle around that bullet.

My previous 7 WSM handled the 180s in a 9 twist at 200'ASL. Can't remember how fast I was running them with 7828ssc.

Yup, no downside to an 8 twist.

The factory barrel on this rifle (Rem 700) wouldn't stabilize long bullets, key holed 168 Barnes LRX's and the 180 Scenars were no better with any load.

+1

Why not.

DF

I built my heavy 7 WSM with a 9 twist; this was before the 195-gn Bergers came out. My rifle does shoot the 195's well (24" barrel) but that was not a given... any future big 7's will be an 8 twist.

Exactly, no down side to an 8 twist 7mm mag.

I appreciate all the input. I know that shooting through this hot soup we call air at slightly above sea level the heavier bullets don't do as well as they do at 7000' and 14% humidity

You know humid air is less dense.

I did not know this, thank you. For more...
http://www.theweatherprediction.com/habyhints/260/


"The amount of water vapor in the air also effects the density. Water vapor is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor increases, the amount of Oxygen and Nitrogen decrease per unit volume and thus density decreases because mass is decreasing.

The two most abundant elements in the troposphere are Oxygen and Nitrogen. Oxygen has an 16 atomic unit mass while Nitrogen has a 14 atomic units mass. Since both these elements are diatomic in the troposphere (O2 and N2), the atomic mass of diatomic Oxygen is 32 and the diatomic mass of Nitrogen is 28.

Water vapor (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 1 + 1 + 16 = 18 atomic units. At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become.

You may be familiar with the concept that moist air is less dense than dry air. This is true when both have the same temperature or when the moist air is warmer. Said in another way, air with a greater percentage of water vapor will be less dense than air with a lesser percentage of water vapor at the same temperature. Often people erroneously believe that moist air is denser than dry air because very moist air is more difficult to breathe than dry air."

1in9, 2900fps 25' ASL completely stable

For the uninformed, what is the downside of a too fast twist with say medium weight for caliber bullets?
Also looking into aftermarket 7RM and thinking 1:8, will 150-168s work?

There is none. If RPM gets too high you can start zooking frangible bullets, mid-flight. Think fast-twist .22-250 and light-weight Horn SPSX bullets. Other than that, no problem.

Yes.

As the Canadian said....None.

Check out Berger's twist calculator. It's a great resource.

Thanks,
sorry for the hijack, back to normal programming.