marksman L1 shock

It applies to the existing ammo type. For example if you're firing an AP Rifle and you have MML1, the ammo type becomes AP+Shock.

 

It applies to existing ammo types if there is one that targets the same attribute, otherwise it is added separately. Shock + DA combines to yield two ARM saves that both cause instant death to 1-wound troopers. Shock + Breaker targets different attributes and as such do not combine, yielding one ARM save that causes instant death to 1-wound troopers and one BTS save at half BTS that does not cause automatic death to 1-wound troopers.

Also remember that Marksmanship is optional, you can shoot using only Marksmanship L2 and eschew Marksmanship L1 in order to not give e.g. a Bolt (Bioimmunity) better saves.

http://infinitythewiki.com/en/Types_of_Special_Ammunition

COMBINING SPECIAL AMMUNITION
Weapons with two or more Special Ammunition types in brackets separated by the 'plus' symbol (AP+DA, for example) apply the effects of both in each of their Attacks.

Certain Special Skills, such as Dual Wield, allow the user to combine the effects of two or more types of Special Ammunition.

In either case, when the combined effects involve the same Attribute (ARM, for example), apply both in the same Roll. For example, in the case of AP+DA, two Special Ammunition types that affect ARM, each successful attack would cause the target to make two ARM Rolls (due to DA Special Ammunition) with his ARM value halved (due to AP Special Ammunition) in both Rolls.

Otherwise—if the Special Ammunition affects different Attributes— then their effects are not combined, but applied separately, forcing the target to make a separate Roll for each affected Attribute. For example, in the case of AP+E/M, the target must make a halved ARM Roll (due to AP Special Ammunition) and, additionally, a halved BTS Roll (due to E/M Special Ammunition).​