Thread: "Air space" in 303 British and 308 Winchester caseloads...Dangerous?

With reloading 303 and 308 I am now looking at reducing the projectile seating depth to reduce the "bullet jump" I currently have with commercial loads. Sadly, there are 3 gray areas of my knowledge.

1...Having entered reloading via black powder, I am very conscious of the danger from air space in BP rounds and with the reduced seating depth of 303 and 308 projectiles, is the increased airspace in the case a danger, as it is for B.P. I am using when using ADI AR2208.

If not, great, if so, would the usual space fillers for B.P. be suitable?

2...Reducing the projectile seating depth, results in less length of the projectile in the case. For safety and pressure, is there a minimum length of projectile required in the case.....or is it that it must just be solidly fixed?

3...Does the answer to 2 also apply to boat tail projectiles as they have a tapering end which, dependent upon the taper, reduces the ability of the case to securely hold the projectile?


Finally does the OAL measurement as given in the appropriate dies have any relevance to the above? i.e not to be exceeded under any circummstances.
Thank You.

The extra air gap wont be a problem in that situation.
There is a risk of the powder detonating rather than burning in very reduced loads tho....like less than 60% case fill.
Rule of thumb is the bore diameter of engagement but in reality nowhere near this is required.
If you have a good 3-4mm contact in the neck you should have no problems in a bolt action.

Don't use fillers.

1. Give a factory round a shake and listen to the powder rattle around. A 100% full case is ideal for consistent ignition and chasing very low extreme spreads, but it's perfectly normal to have less than 100% and a little airspace.

2. One full bullet diameter is a good guideline. Some go less, 2/3rds or even 1/2. But more will aid in concentricity and give increased tension which is kind of what you're after with hunting ammo.

3. The boat tail isn't measured in the above, so for a boat tail bullet the guideline above would read "seat the entire boat tail plus a full bullet diameter".

It's fine as long as you aren't playing around with reduced loads with really low density.

I think the rule of thumb is that you should have at least a bullet diameter worth of projectile in the case neck.

Gotta be quick with answers around here!

Originally Posted by Pommy

Don't use fillers.

1. Give a factory round a shake and listen to the powder rattle around. A 100% full case is ideal for consistent ignition and chasing very low extreme spreads, but it's perfectly normal to have less than 100% and a little airspace.

2. One full bullet diameter is a good guideline. Some go less, 2/3rds or even 1/2. But more will aid in concentricity and give increased tension which is kind of what you're after with hunting ammo.

3. The boat tail isn't measured in the above, so for a boat tail bullet the guideline above would read "seat the entire boat tail plus a full bullet diameter".

Gotta be quick with answers around here!

Yup Pommy summed it up in a nutshell, concentric rounds are more important than jump in my book. But that's just me.

Originally Posted by Kiwi Sapper

With reloading 303 and 308 I am now looking at reducing the projectile seating depth to reduce the "bullet jump" I currently have with commercial loads. Sadly, there are 3 gray areas of my knowledge.

1...Having entered reloading via black powder, I am very conscious of the danger from air space in BP rounds and with the reduced seating depth of 303 and 308 projectiles, is the increased airspace in the case a danger, as it is for B.P. I am using when using ADI AR2208.

If not, great, if so, would the usual space fillers for B.P. be suitable?

2...Reducing the projectile seating depth, results in less length of the projectile in the case. For safety and pressure, is there a minimum length of projectile required in the case.....or is it that it must just be solidly fixed?

3...Does the answer to 2 also apply to boat tail projectiles as they have a tapering end which, dependent upon the taper, reduces the ability of the case to securely hold the projectile?


Finally does the OAL measurement as given in the appropriate dies have any relevance to the above? i.e not to be exceeded under any circummstances.
Thank You.

Greetings Kiwi Sapper,
Loads down to start loads will be no problem. With AR2206H you can go down to 60% of max loads for the cartridge and projectile. Best performance will be somewhere above that. I have found in the two cartridges you mention you can still get excellent accuracy with the projectiles well short of the lands, especially with flat base projectiles. Boat tails can be a bit wonky. As mentioned above 1 calibre is the accepted minimum but I have loaded down to 6mm in .308 and .303 with good results. Your magazines may dictate your LOA. A longer jump to the lands will generally lower pressure. I have found that the Hornady 174 grain round nose projectiles are excellent in the .303.
Happy loading Grandpamac.

Rule of thumb, is , dont use your thumb, use a bullet daimeter. If your in a boat still use the thumb and make sure your tail is covered to prevent detonation if your less than 60 % full.
Oh hang on I might have that back wards

Originally Posted by mimms2

I'm still cautious with BP (My method is "fill it")
However, from every verfiable source I've ever come across, air gap is another unfortunately persistent myth.
Dacron is generally accepted, it wont absorb moisture. Some peaople use "cream of wheat", powdered oats etc.

The general consensus with smokeless is fill >80% case volume.

There is absolutely NO evidence anywhere (not, "mate of a mate") That reduced smokeless loads are dangerous at all. Laborotories have tried and failed, to prove the myth of underloading. My diagnosis is that people either double-charge, or pull the trigger on an obstruction (the previous round, didn't clear the barrel) But I have no evidence for this.

General consensus is "one calibre" holding projo in neck. .303 ideally wants 7.62mm held by the neck.
Most trad 303 have the leade shot out and you won't be able to seat to the lands anyhow.

OAL depends on projectile and a more useful measurement is "Base to Ogive"

@mimms2 I thought I read years ago that they did prove it?
Took a certain set of parameters
A certain powder type, in a certain size case as in maybe 308 or bigger, and massive underloading like less than 50% of volume. Something like that
Then it was relatively repeatable.
Not generally a few grains under minimum but quite a lot like a cock up with the powder thrower or misinterpretation of load data. @grandpamac do you remember the details or is it indeed a myth

What Ho Chaps,

My thanks for your helpful and reassuring posts. I now understand that:

Case neck to projectile contact should be the diameter of the projectile.......Although this precludes my efforts to substantially reduce bullet jump in my 308. So be it. I asked, you answered, I follow.

As I seek "hot loads like a gay guy seeks a couger," and as an x civil servant, "I can follow the handbook instructions", <80% low loads are of no interest" SO filling "air space" with modern powders will not be an issue.


The "Boat tail" comments are particularly helpful and reassuring. I only have a left over 80 pack of them which I will save for the rainy day I runut of FMJ for the Spanish FR 8 which is #2 308.

Just a couple of "things'..........

Thing 1......I learned a new word today....concentricity I Googled and apart from understanding a central point of a circle....I am still Duh and fail to understand the link to projectiles...apart from being round and "balanced'


Thing 2......I still lack guidance on my query "does the OAL measurement as given in the appropriate dies have any relevance to the above? i.e not to be exceeded under any circumstances"

I understand @mimms2 post ( Thank You) "..OAL depends on projectile and a more useful measurement is "Base to Ogive" as homing in on "bullet jump" i encountered another new word Ogive and I have acquired the goodies required to measure that and it is the measure for improvment ......even if I can't achieve it because of chamber length.

SO, is OAL a safety Law or guidance?


Again, many thanks.



P.S. @grandpamac....

I followed your guidance re projectiles for the Parker Hale Mauser 308 and now have a goodly collection of Hornady Interlock 150 flat base . The 80 BT 's are leftover from seeing which she liked :>)

Originally Posted by csmiffy

@mimms2 I thought I read years ago that they did prove it?
Took a certain set of parameters
A certain powder type, in a certain size case as in maybe 308 or bigger, and massive underloading like less than 50% of volume. Something like that
Then it was relatively repeatable.
Not generally a few grains under minimum but quite a lot like a cock up with the powder thrower or misinterpretation of load data. @grandpamac do you remember the details or is it indeed a myth

My understanding is that detonation has never been satisfactorily proven.
Problem is running any evidence to ground.
There are lots of other forums out there with threads on the topic, and lots of heresy.
Lots of theories around specific powders being the cause.

I personally run a massively reduced load in one of my K31 Swiss rifles.
I use Haendler & Natermann swaged lead projectiles.
The load from their reloading tables states no filler required.
I was a bit leery at first about the load etc but have fired maybe a hundred rounds, and all good.
I did contact H&N to confirm the no filler needed bit.
The accuracy is v good and the velocity ES is reasonable with no oddball spikes.
The powder used is not a bulky/case filling powder.

As I understand it going longer than the OAL means it won't feed or probably wont feed from the magazine.

There must be other reasons but being new to all this I don't know them.

Cast lead is a bit of a different matter. For me 10 grains of blue dot certainly is nowhere near a full case but under a 180 grains cast lead projectile my martini enfield loves it. No filler. Actually a full case of blue dot would be very bad i suspect.

had 110grn loads in 270 made for me by local sports shop using 760 powder...but me thinks it was old/dodgy batch....UNLESS I did the point at moon.lower and fire thing,I got double detonation...I had asked for velocity around 3000fps.... by putting powder in rear of case the detonation was good.
years later I did the maths and tried to duplicate this load with ZERO issues.
found a hard case way to seat to projectile width the other day..pretty much by mistake.... I placed projectile in case mouth by hand,screwed die back out,took it down till it just touched projectile with projectile sitting sideways between base of die and case holder,tightened things up....removed sideways projectile and proceded to seat....seemed to work well... NORMALLY all I do is set die with a factory round...or previously made reload...place loaded round in shell holder,unscrew top of seating die,lower ram,then slowly screw stem inwards till handle twiches like your old fella when is sees boobies...STOP thinking of boobies....stop threading stem in....and proceed to seat your loads...check first one out of press in mag and chamber...WITH ALL DUE CAUTION...

Originally Posted by Kiwi Sapper

With reloading 303 and 308 I am now looking at reducing the projectile seating depth to reduce the "bullet jump" I currently have with commercial loads. Sadly, there are 3 gray areas of my knowledge.

1...Having entered reloading via black powder, I am very conscious of the danger from air space in BP rounds and with the reduced seating depth of 303 and 308 projectiles, is the increased airspace in the case a danger, as it is for B.P. I am using when using ADI AR2208.

If not, great, if so, would the usual space fillers for B.P. be suitable?

2...Reducing the projectile seating depth, results in less length of the projectile in the case. For safety and pressure, is there a minimum length of projectile required in the case.....or is it that it must just be solidly fixed?

3...Does the answer to 2 also apply to boat tail projectiles as they have a tapering end which, dependent upon the taper, reduces the ability of the case to securely hold the projectile?


Finally does the OAL measurement as given in the appropriate dies have any relevance to the above? i.e not to be exceeded under any circummstances.
Thank You.

I suguest you buy a reloading manual preferably put out by the manufacture of the projectiles you are using, load your ammo with the overall length as given
and the start load and work up in 0.5 grain steps until you find the most accurate powder charge this will give you a known load to check other loads performance
against, choose a powder that gives the best velocity with 90% case capcity compressing modern smokeless powders is not recommended when starting out,
With the .308 and .303 Brit 2206H and 2208 are the easiest to work with both lead and jacketed bullets for reduced loads use Trail Boss, Stay within recommended
powder charge weights and you can't go wrong, And forget about S.E.E it does not happen.