Mil. R. Evid. 106, is a rule of completeness and applies to both the prosecution and the defense.
Rule 106. Remainder of or related writings or recorded statements
When a writing or recorded statement or part thereof is introduced by a party, an adverse party may require that party at that time to introduce any other part or any other writing or recorded statement which ought in fairness to be considered contemporaneously with it.
Federal Evidence blog has reported a Second Circuit opinion dealing with Fed. R. Evid. 106 (the same rule as the military rule).
Second Circuit concludes the rule of completeness did not require the admission of a defendant’s redacted statements, which were offered to show he lacked the intent to kill, since the statements were not relevant to the charges, in United States v. Kopp, __ F.3d __ (2d Cir. April 6, 2009) (per curiam) (No. 07-2797-CR).
I think the same result would happen under Mil. R. Evid. 106. And the conclusion that the accused would have to consider testifying to get the statements in would be correct. However, for the military accused the inquiry doesn’t stop there. A point often overlooked by Trial Counsel and sometimes defense counsel is Mil. R. Evid. 304(h)(2).
(2) Completeness. If only part of an alleged admission or confession is introduced against the accused, the defense, by cross-examination or otherwise, may introduce the remaining portions of the statement.
There is no requirement of relevance in the rule, and it is a rule for the defense. There may be a Mil. R. Evid. 403 objection, but if Kopp were at court-martial his statements would more likely come in under Mil. R. Evid. 304(h)(2). There is no federal equivalent of this rule.
Remember that under Mil. R. Evid. 1106, federal rules are adopted 18 months after enactment for courts-martial, unless the President declares otherwise. Also, the military rules have a number of provisions or additions not found in state or federal evidence rules.
As always I commend Federal Evidence blog to you as a good resource.