I found it, but it seems it was the Cockman Family, not the Carter Family who recorded it. Stupid site keeps erasing my post when I go back after more info. From: Pentecostal Hymnal page 23 of Pentecostal Hymnal On Line. Warning: Pop Up Ads there!

Keep On The Firing Line
American traditional

1. If you're in the Battle for the Lord and right
Keep on the firing line
If you win the victory, brother, you must fight
Keep on the firing line
There are many dangers that we all must face
If you die still fighting it is no disgrace
Cowards in the service should not have a place
Keep on the firing line
You must fight, be brave against all evil
Never run, nor even lag behind
If you would fight for God and for right
Keep on the firing line

2. God will only use a soldier He can trust
Keep on the firing line
If you'll wear a crown then bear the cross you must
Keep on the firing line
Life is but to labor for the Master dear
Help to banish darkness and to spread good cheer
We shall be rewarded for our service here
Keep on the firing line
You must fight, be brave against all evil
Never run, nor even lag behind
If you would fight for God and for right
Keep on the firing line

3. When we get to heaven we shall be so glad
Keep on the firing line
We shall praise the Savior for the call we had
Keep on the firing line
'Twill be joy to see the many ransomed men
Those who came to Jesus from the paths of sin
Hear their happy welcome and go marching in
Keep on the firing line
You must fight, be brave against all evil
Never run, nor even lag behind
If you would fight for God and for right
Keep on the firing line