Sierra Blitz King real BC

[wabbit evaporator]

Does anyone know where I can find something approaching the real world BCs of .20 cal. SBKs? 32grn and 39grn? The manufacturers numbers don't really add up!

[garyw]

Does anyone know where I can find something approaching the real world BCs of .20 cal. SBKs? 32grn and 39grn? The manufacturers numbers don't really add up!

And they wont add up-when i had a .20 tact i ran the numbers for the 39gn BK the data didnt match my real world use -so i just worked out to 400yds in 50yd stages and tested accordingley.i dont think i ever shot the rifle beyond 350 at vermin and the real world data was spot on everytime.

[wabbit evaporator]

Fair point Gary, I was just being lazy! Will get out there on Saturday!

[tisme]

I use .209 for the 39gr BK works fine for me

[wabbit evaporator]

Thanks Tisme.

[Andrew]

Why don't you guys get together some weekend with a Chronograph and calculate the actual BC for your location and environmental conditions? That is the only time you will have the "real" BC of any bullet. .~Andrew

[Big Al]

I use .209 for the 39gr BK works fine for me

 

That seems very low?

 

Then again starting velocity will play its part, what cartridge is it coming out of and at what MV?

[tisme]

 

That seems very low?

 

Then again starting velocity will play its part, what cartridge is it coming out of and at what MV?

Tried and tested a few times out to 600 yards and my first thoughts were it was low but seems spot on. MV is 3454fps

[Big Al]

Tried and tested a few times out to 600 yards and my first thoughts were it was low but seems spot on. MV is 3454fps

 

Your MV could be the reason for a lower BC I would expect. Then again is the scope centre height current and are your click values really 100%?

 

Im running them at 3750 in a 20TAC and 4000 in a 20BR - a BC of .245 is more like what Im getting, verified out to 400yds. I even wonder if twist rate as well as velocity will influence the BC value we end up with?

[247sniper]

Does anyone know where I can find something approaching the real world BCs of .20 cal. SBKs? 32grn and 39grn? The manufacturers numbers don't really add up!

 

 

I have calculated my own G1 Standard bullet BC for my .204 . Make sure you account for atmospherics, measure the range between two or more chronos as carefully as possible. Set up the chrono's in close tandem to make sure they're synch'd up meaning, they measure the same speed for the same shot out of the muzzle.

 

Testing as described will give you an accurate G1 BC over the range of velocity that your bullet sees in that test. Outside that velocity band, your G1 will be different. If you determine the bullets G7 BC in such a way, it will hold better over a wider range of speeds. This is what I have done.

 

If you need factual real life figures let me know bud.

 

Steve

 

[Laurie]

I even wonder if twist rate as well as velocity will influence the BC value we end up with?

 

 

It can. An inadequately stabilised bullet can perform excellently in terms of round holes and group size, but still lose out of BC. For small calibre varmint type bullets, the key value appears to be ~1.30 Sg (Coefficient of stability). Below that, the quoted BC value is way overstated and even in the 1.3 to greater than 1.4 bracket, some BC is 'lost'.

 

Latest thoughts on BT form larger calibre bullets is that full ballistic stability is only achieved at 1.5 Sg, so whilst the .308 Berger 185gn LR BT 'Juggernaut' is known to 'shoot well' in 1-13" twist TR and FTR rifles, its actual in flight BC is degraded by 9% - a large enough drop to negate any benefit of this bullet over a fully stabilised high-BC 155gn at 3,000-3,050 fps. (An 11.5" twist barrel is needed to achieve 1.5, but a 12" barrel is very close and only sees a small BC reduction - a caculated 3% at 2,800 fps.)

[Big Al]

 

 

It can. An inadequately stabilised bullet can perform excellently in terms of round holes and group size, but still lose out of BC. For small calibre varmint type bullets, the key value appears to be ~1.30 Sg (Coefficient of stability). Below that, the quoted BC value is way overstated and even in the 1.3 to greater than 1.4 bracket, some BC is 'lost'.

 

Latest thoughts on BT form larger calibre bullets is that full ballistic stability is only achieved at 1.5 Sg, so whilst the .308 Berger 185gn LR BT 'Juggernaut' is known to 'shoot well' in 1-13" twist TR and FTR rifles, its actual in flight BC is degraded by 9% - a large enough drop to negate any benefit of this bullet over a fully stabilised high-BC 155gn at 3,000-3,050 fps. (An 11.5" twist barrel is needed to achieve 1.5, but a 12" barrel is very close and only sees a small BC reduction - a caculated 3% at 2,800 fps.)

 

 

Thanks for that Laurie, as I guessed, everything that has an influence on the bullet will also in some way play its part in determining the actual BC.

[gbal]

Thanks for that Laurie, as I guessed, everything that has an influence on the bullet will also in some way play its part in determining the actual BC.

Al,it's generally better to start from that point than be continually disappointed. I feel for poor old BC, wondering when history will repeat and it moves into AD.

BC (Basic Calculation) has helped,especially with it's recent facelift to G7,revealing the vagaries of the G1 which has caused much mischief in ballistics programs when 'the numbers don't add up" (though there are several other contributors there too).We are moving close to the age of AD (Advanced Dynamics).Laurie outlines Gyroscopic Stability (SG),and this relates to twist rates and velocity,as he shows in an example.

 

SG is a measure of bullet stability.Forward velocity decreases downrange faster than rotational speed (spin),though the bullet will remain reasonably stable downrange,if muzzle stable,until it gets into the notorious trans/sub sonic zone (1180 fps) when stability becomes vulnerable (308s eg can get dodgy with some (168) bullets around 800y or so-though are excellent at shorter ranges).

 

Dan Lilja has shown that the 'standard' SG of 1.0 means that a bullet is only just stable,below that it is ever likely to yaw and be inaccurate. Above 1.0 SG,it will shoot clean round holes,and may well group very well,though SG 1.3 is more generally aimed for as a minimum for reliable accuracy and performance,as below that the bullet remains vulnerable to externals-such as an imperfect crown,fouled bore,and environmental factors (see below)-note these externals didn't much concern Bench Rest shooters who didn't have dinged muzzles ,and cleaned often etc.

What effects do environmental variables have-well,in UK we are exempt from the extremes but here is an example:

Humidity 90 and altitude 750' ,no other changes but temperature changes from 100F to 30F:

a 14 twist barrel giving very good SG of 1.74 can drop to a marginal 1.26 SG at the lower temperature

This is for a flat base bullet-boat tails and length decrease SG even more.

 

Any better news ? Well,try for SG 1.5 or more,within the twists offered;though 'too fast' does diminish accuracy very slightly in a BR quality rifle,it is safer to err if possible on the side of 'fast' twist,based of course on the likely prevalent conditions-as said,UK is a bit less extreme in temperature,and altitude etc-so obviously factor in UK conditions (and send your "it ain't half hot and high,Mum" PCs when on the Raton,NM ranges).

 

Better yet! Some of the $ spent on Berger bullets have reached that nice Mr Litz,and there is a free online Berger Twist Rate Stability Calculator available. Bryan argues too,that BC can increase with twist rate (Sierra actually give figures for BC1s at different velocities-don't miss that if you have to use BC1s).

It follows that if you are not spinning your bullets fast enough,your effective/actual BC may be lowered,with bullets dropping and deflecting more than expected ("the numbers don't add up'?).

The gyroscopic factor SG,on average ,correlates .87 with BC;and BC drops off quite rapidly as GS drops below 1.5

 

G7 reductions can vary from 1.5 % to a whopping 20% (when twist rate is not optimal-even though acceptably stable).

 

An (average) ball park figure is that BC drops by 3% for every .1 that SG falls below 1.5 (average since it does vary a bit with bullet design etc). 10% would mean your BC G7 .25 is actually .225; or G1 might be .36,not the expected .40).Enough to matter. ('not add up').

 

Great groups and circular hole(s) are a delight,of course,but do not guarantee that the bullets' full ballistic "BC" potential have been realised,and if not,it will show downrange (won't add up,down,or sideways).

 

(the shorter range shooter can still take considerable comfort from the 100/200y Bench Rester,without a chrono,who said "If they all go through the same hole,who cares how fast?") Sanity,not vanity...as far as it goes ..... (only about 250 yards) :-)

 

Dan Lilja: Effects of Altitude and Temperature on (optimal) Rifle Twists.(google)

 

Bryan Litz: How BC Varies with Twist Rate (Accurate Shooter site)

 

Checker,Chubby "Do You Remember When..." backed by "Let's Twist Some More" BC/AD Records

 

 

gbal

[gunner]

Hi w-evaaporator , i found that like many others the 39 sbk has a 250-260 bc from my outfit , but as al has stated every rifle / barrel etc will be different , iv not bothered with 32s , from my 20 practical there doing 3652 fps , in a 9 twist 25½ " tube , similar i guess to a 20 tactical . Hope this helps , atb ...